What I normally do is just level up a bit and then purchase ZP, but that's just me.
If you do not plan on buying ZP, here are some things I would do.
a) Buy an M4A1 to start off and use that for a while. When you get promoted a few times, buy a Desert Eagle, and eventually buy an AWM.
b) Play Elimination (EM) until you are a decent rank. That way, you do not have to buy/repair guns. Therefore you will be saving GP. Once you are a decent rank, purchase some higher class weapons such as the AWM or Desert Eagle, or a fully automatic if you would like. You can easily save up 100k GP this way.
Have fun and good luck!