The most popular pistols (in descending order): G18, Shorty 12g, Deagle. You're at a huge disadvantage if you use anything else. At close the Glock's full auto has a short TTK, the 12g can be a 1-shot and the Deagle can be a 2-shot. The M9 is a very good pistol if you want something else

Tips: Learn how active protection works & when to time it to defend yourself, higher bullet velocity = less target leading and drop (super important for choosing a DMR and sniper), smoke grenades are OP for creating 'cover' when you need to move or hide but also you can throw a smoke to fool the enemy into thinking you're hiding somewhere you aren't. Most of the DLC weapons are really good so complete the assigs to unlock them (AWS, Bulldog, AS Val, GOL, MTar, MPX and the non-dlc assig weapons like the ACE23, UMP9)