A couple of opening notes. I haven't really seen any general, novice-oriented guides floating around; then again, I am rather new to the forums. So here's one.
I'm not a particularly outstanding player, but I do consider myself at least decent. This is a hopefully helpful, but in no way comprehensive, collection of tips for beginners. It's a guide I wish I had when I was a beginner. It's a guide that is not class-specific, allowing a neophyte to retain a sense of experimentation essential to his or her growth as a gg2 gamer. It's a guide that attempts to turn a beginner into a decent, likable player.
Welcome to the entry level. Let's get started.
1) Beginners should choose a useful class.It's okay to play around with a variety of classes. Hell, you really can't get a full gg2 experience without playing with all the classes. But, if you are a beginner, it would be wise to start off by maining a class that both has a generous learning curve and is useful to your team, even at relatively low skill levels. In my opinion, the three classes that have the highest learning curves are:

These three classes are rather unfriendly to beginners. They all require skill and experience to properly play. On an unrelated note, some of the best players I know main

or

because they are bloody powerful in the right hands. However, a beginner

is likely to plant mines and sit, not playing dynamically or using the Detonator to its fullest extent. A beginner

is usually very predictable and therefore fragile. And there is little in the game more infuriating than being on a team full of failspies.
The lesson here is that if you haven't yet found a class that you like or are good at yet, play a class that is instead useful to your team. Good candidates include:

That's not to say that playing a good Healer, Constructor or Overweight doesn't require any skill. It still does. However, any

is likely to be useful to its team; more useful than a beginner

, at any rate. Who knows? Maybe in a couple of weeks (
days for some of you freaks), a beginner might become a really, really good Healer. Be careful with the

, however. It's only a useful class insofar as a well-placed autogun is almost always useful. Learn proper autogun placement. It shouldn't be too hard with some experience.
2) Be dynamic.Have you ever seen a pro play? Pros are always on the move. A pro

will never stop changing direction. A pro

(aren't these things supposed to be
easy to hit?) will never stop dodging rockets. Pro Riflemen will never stay in the same place. Pros will also keep changing their game, keep being unpredictable and keep utilizing situational tactics. And then they'll switch to another tactic that better suits a new situation. And then they'll screw you over.
The lesson here is to NOT be stationary or predictable. An easy way to survive longer? Dodge bullets. And what if your sneaky opponent predicts that you will dodge those bullets? Change directions. Simply moving around a lot will do wonders to your survivability. Once you have the concept in your head, the rest is experience (i.e. learning how to dance with

or detojump or other such movement-related skills). And don't be afraid to retreat! Another good way to live longer is to know when to step back. If you're outnumbered or outmaneuvered, you'd better get out of there to regroup with your teammates or heal up. Don't be one of those guys that tapes down the "forward" button, and definitely don't be a rock.
3) Defending is good.Speaking of retreating, it is often the case that the player with the defensive tactic (moving backward) has an implicit edge against the player with the offensive one (moving forward). Bullets fired when moving backward stay in the air longer; an approaching opponent may simply run into them. A backwards-moving

has a longer flame trail than normal. A backwards-moving player can use his or her surrounding terrain against the opponent, leading the opponent into corridors or other such deadly situations (a tactic a good

will exploit). Additionally, the more you move backward, the likelihood of you seeing reinforcements increases and the likelihood of you seeing more enemies decreases.
The lesson here is to be smart about defending. A mistake that tragically many beginners make is the classic Leeroy Jenkins, a super-aggressive mindset that leads to lots of deaths and lots of team frustration. No one will get annoyed at you if you retreat a lot; no one but your opponent, who will be too busy being killed to notice.
Another mistake beginners make is the stagnant, non-dynamic defense. A

who hides in his spawn, not moving, planting stickies and waiting for opponents to come is not a strong defense. This is because it is very predictable and not dynamic; the opponent knows exactly what you are going to do and will therefore find a way to circumvent it. The same applies to stationary

players, who will build an autogun and then stand behind it, unmoving, spamming bullets. Predictable defenses are
not smart defenses. Combining dynamism with a defensive mindset is a good overall strategy for a beginner player.
The best defense isn't a good offense. That's stupid. The best defense is the best defense.
4) Be aware of your team.At its heart, gg2 is a game that pits two teams of players together. The better team wins. Not the team with the better players; not the team with the better arrangement of classes. The better
team, as a whole, is the team that will win. One of the easiest ways to stop being a beginner - or at least not appear like one - is to be aware of your team at all times and to play with it. Do you already have 2

on your team? 9 times out of 10, it is a good idea to not play as an Infiltrator. Do you already have 2

or 2

? The same thing applies. Most good teams have a variety of classes in order to facilitate flexibility.
Another element of team awareness is communication and the position of the team members around you. If 6 out of 8 of your team members are busy pushing to Checkpoint B, don't place an autogun and defend Checkpoint A. If you're a

preparing to force your way into an enemy stronghold and a

is a couple of seconds behind you, wait for the healer to catch up to you so the two of you can attack as a unit. If you see a lone Infiltrator headed into enemy territory, don't stay close to him. Synergy is in the simple decisions that add up to have a large impact on overall team performance.
The lesson here is to be a team player. If you're a

then heal others. If you're planning an offensive push, team up with another player. It's common sense, but one of the biggest things that separates beginners from decent players is teamwork.
5) Know the win conditions of each game mode.In Boner's Server, there's a reverse intel map that requires putting your intel into the enemy intel point. The instructions are on the map. And yet, in many games I play on that map, I always see some player take the team intel and bring it all the way back to the same team's intel point, reversing all the progress of the team. Even more infuriatingly, sometimes they hold on to the intel and continue trying to somehow shove it back in to their own intel point, not knowing that it will never work and they are only slowing the entire team down.
The lesson here is to learn how to play the game modes. Now, given, the reverse intel map is rather unique. However, in an intel map, the win condition is to get more intel than the enemy can before time runs out or to get a certain amount of intel. This leads to two logical ways to be useful in such a map: attempt to capture the enemy intel or defend your own. Most players do this. That is good. Some beginners, however, camp out in some arbitrary location (sometimes it's the enemy spawn, sometimes it's just a middle path) and try to kill anyone who comes across. The helpfulness of such a decision is very limited, and the player would be put to much better use defending at his point or going for the intel.
Similarly, players should defend or attack in generator maps, and they should GET ON THE DAMN CONTROL POINTS in CP maps. If you are playing an

then uncloak and STAND ON THE GODDAMN POINT to help your team capture points.
6) Use but do not abuse alternate attacks.Most classes have an alt attack or a special ability. Use them. All abilities in this game are useful and all of them can be very beneficial in the right situations. You'd be surprised at how many beginner

players never Ubercharge or how many beginner

aren't even aware of rocketjumping. Perhaps the most notorious example is the Airblast. I die a little inside every time a beginner

gets splattered by an easily avoidable rocket. The airblast can also put out fires that are damaging your allies or push enemies away from you; unfortunately, all of these useful abilities are almost never used by beginners. All alt attacks are useful; players should learn them and use them, given the proper opportunities.
Even more annoying but no less detrimental is the abuse of alternate attacks. An

plopping down in the middle of the battlefield to eat a sandwich is a veritable sitting duck. A

running into a mob of enemies spamming needles is funny for about 2 seconds and sad for the rest of the time. Again, we have a notorious example. A

who does nothing but stab is wasting one of the very best weapons in the game - the almighty revolver. The fact is, all alternate attacks are alternate for a reason: they are situational. They provide useful abilities, but only in their proper situations.
The lesson here is to use alternate attacks as they should be used. It takes a bit of learning and experience, but it will improve one's game significantly.
7) Don't namedrop on forum guide posts.It's a pretty good way to upset forumgoers, as it turns out.