Learn to use M-82 and the scopeless machine gun. Also, learn to use both without zooming, firing only from your hip. Those are the best beginner weapons. You can also experiment with pew-pew as well, on bigger maps. I advise against using the SMG, because although it is devestating in close range, it has certain flaws that makes it's usage quite hard on certain occasions. Also, if you have time, get the revolver. It's an extremely powerful weapon if you know how to use it.
Play on maps with a maximum of 16 players, with the assault class completely shut off (to prevent the usage of imba boost and the inevitable shotgun+boost players). Stick to your region. Lag can easily brake your experience (because you WILL fail at shooting targets not to far away and ask yourself how the hell did you have 3:1 KDR in the last game and you can't even maintain proper 1:1 in this one). Lag is your friend only if you use the shotgun (and, ESPECIALLY, if you use shotgun + boost).
If you want team play, stick with clans. Team play is something that you will find very rarely on random games.
Don't use melee on maps with a large amount of players. The lag will make your melee hits very rare and it will be practically useless.
Mute everybody (or, if you want to have a slight advantage, don't mute your opponents, because their chat will also reveal their position). Use custom soundtrack.