how do you win in c.o.d Modern warfare 2?

this game is driving me insane I had it for about a week now.(level 51). I can’t win at all. in free for all I get pound on like a nerd. and Team deathmatch my team alway do the winning for me( not really fun). ect…I try to be a sniper didn’t work I try to be a Knifer I did ok at it.. but as far as shooting go. I mostly get own. I can be sneaking around and boom am dead.. why is this game so impossible to be good at. like all my other games am good at. ( I was even a world top ranked in Ufc).. it don’t help that there mega noob in MW2. using Akimbo Ranger or models. or noob tuber..I guessing am not the only one who feel the same about this. cause online tons of peoples be mad. ( and racist). I try switching up my perks I try switching up my guns. it just seem like everyone is just to good. I alway be like 1 kill alway from my first Killstreak reward. and boom some * way someone get to me and kill me. like seriously. what B.S…then you got the Mega campers -.- ugh don’t even get me started on them. so yea I need some advice on how to be good online…well am about to go play it right now :D thank