how do i get better guns for multiplayer online?

sorry this might be a dumb question as i have never played any call of duty games before and i havent done the campaign yet.

when you play online there are only a few guns to choose from like the kuda and some others. i notice when i play online and kill someone and take their weapon that it is a lot better than mine. is there more selection for weapons and accessories as you level up or do you have to do the campaign to unlock more stuff.