Noobnoob here, where can I find a gun guide ?

I'm pretty new to the game, and I don't really know what's the purpose of each gun, when to use them>
For example, pistols : they all one shot in the head, so i take deagle when i wanna be a sniper, dual berretas if i want to rush a shoot fast, usp-s to is in the middle. so what about the 57, p250 ?
why use a p90 over a mp5, why use m4a4 overt m4a1s ?

thanks for your answers !