How do you level guns in Team Fortress 2?

Ok, I've had this game a very long time, and every where I go I see a level 90 flare gun, a level 42 natasha. (Non-gamers, feel free to ignore this post). When I ask in game, I never get a straight answer. I know this post may sound a bit feeble and not very much worth posting, but can someone enlighten me?