Well, make sure you know what gun you will like... If you want a spray-and-pray gun, go for the M4 family. If you like tap firing (which is always better) go for the G36 or AK family. If you want to be unique, don't get the popular guns (M416 CQB, G36e Valkyrie, etc).
For tap firing, the AK74M is amazing, but so is the Valkyrie.
For spraying, the CQB is awesome due to its controllable recoil, but the M417 or Rec-7 are ok too.
Sniper rifles are out of the question because all permanent duration sniper rifles cost 29,900 NX.