There are three stages for each team, which unlock the more advanced weapons and classes. You have to make enough kills to advance to the next stage, before you can get the cooler toys.
And, yeah, don't forget to sell your carried weapon or ckit, so that you can buy a new one. You can't sell your blaster, and the carry limit for "weapon" (which in this case includes the ckit) is blaster plus one. I hope that helps. Somewhere, you can even change the number of kills required to advance the stage counter. I would recommend that you play for a while before changing too many settings, though.
Good luck.
Here's a
link to the manual. Although I'm pretty sure it neglects to mention that you need to sell your rifle, it does mention the carry limit, and there is a brief discussion of stages.
In short: Keep killing stuff. You'll see.
@
kev: lol'd