Check the game threads...arrange for a rental. Chat it up with the guys and have a bucketload of fun.
...afterwards, figure out if you want to run around and get up into the mix of things...or if you want to stand off more and pick people off at range. Personally...there's lots of both to be done, but moving around and getting into it is A LOT more fun for me
If you preferred the run&gun moving around...get a shorter weapon. I.e. M4/G36/AUG/P90/SIG/etc....
If you prefer to stand off...then go for a longer one. DMR/SPR/M16/longer SIG 55x/G3/etc...
A nicely tuned carbine (e.g. M4) will do a nice job of both.
Don't buy anything until you've played a couple of games.
(then...when you get sick and tired of all the pouches/mags/stuff...you'll buy a SAW/LMG