why don't you just play the game you will progress and get better

I'll give you a couple of tips, but watch some popular commentators on YouTube. They give better tips than I can.

1. Stay around the edges of the map. Going in the center of the map, you can get killed from any direction.
2. Don't sprint around corners. You will get sprayed down by someone camping, and it is quicker to aim when walking than running.
3. Work the objectives in objective-based gametypes. Most games I get into, my teammates are really bad and never go for the objectives. So we lose faster than I can get a killstreak.
4. Don't camp, almost always keep on the move. People come back for you, and 9/10 times they outgun you.
5. Use good guns, and good attachments. I suggest never using ACOG, Holographic, HAMR, or thermal. Kick is the best proficiency on most classes.

Try out different guns, and see what works best for you. Good luck!