You have to make Custom Classes. You've been using the top 5 classes on the list, the default classes. Below those are the 5 custom classes you can make. I recommend the M4 until you unlock the SCAR. And the UMP 45 sub machine gun for the smaller maps. and a shot gun for your secondary weapon.
Cool. Free-for-all is a quick way to level up and get more XP. You'll get better once you unlock some more weapons and better perks. A big thing is knowing the maps like the back of your hand and knowing all the tricks you can do in them.
You want to stop getting owned in MW2, stop running down the middle of the streets. Stay next to the walls and get cover. You're an open target out in the streets. I was watching you make these mistakes but couldn't tell you. You need a headset.