Yeah, being fast in Combat Arms is the most important part. If you're a starter, I agree with G36, it's one of the best available current weapons that you could use. One of the problem with G36 that you may encounter is its strong recoil, you will feel it especially when you use the scope.
If you'd like a weapon with less recoil, I recommend M4A1, it has almost no recoil. It has fast rate of fire for an Assault Rifle, but the damage is not that good. If you want average between recoil and damage, try the MK16 SCAR-L.
Tips for saving your GP! If you tend to stick with this game for a while and if you are playing daily, buy your gun for a week. You will save about 30% or so if you buy on weekly basis. Also, don't spend any stupid stuff. Considering you don't buy light vest, don't buy anything else concerning with your appearance or body. I don't believe helmet really helps you that much, so my advice don't buy it and keep your GP in a good condition.
Have Fun