Some tips for a novice player

Hi, I'm sorry to bother you with my post, but could you please give me some tips on how to play TF2 decently? I want to improve. By the way, I'm a Scout main, and I also have some questions, like what I can do with cosmetic cases if I don't want to open them because I don't have any keys yet. I just delete them because they take up inventory space.