First find out what kind of player your are, If you are a long Range player, go with rifles or some times of SMG.
If you are a close ranger, try out with the Shotguns and Smgs. So now that you have a gun, at first try these out on bots to get a feel for them.
Try entering some games and duke it out with some New people.
Remember that SMG's may be a cheap solution, they are not your super close range killers. On the opposite hand, remember The M249 may be expensive, however that expense keeps you from buying other weapons later on.
Get body armor
Get familiarized with your favorite combos and save them.
Sniper rifles may be hard to aim with, but with enough practice, you dont have to buy an AWP to get an instant kill.
Auto snipers are good if you are trying to kill someone from the shadows. THIS IS A COMMON MISINTERPRETATION!!! Never use it as an assault rifle, It may fire automatically, but that just makes it a waste of money.
