Very Easy. Once you reach XP level three, you get an unlock for a weapon of your choice. Works just like CoD and other modern shooters. Pick a weapon, use the unlock, and "pop."
If you plan on boosting this game, I recommend you spend your unlock on the Falcon Blade or rocket launcher. The Falcon Blade has the fewest requirements to get Weapon Specialist, While the rocket launcher enables a boosting method that is very resistant to randoms.
If you're not boosting, just grab whatever fits your playstyle.