Simple tips :-
1. Aim for head. First short on many guns are perfect hits.
2. Stick to a set of guns - pick one in each category (this is for point 3).
3. Learn the spread pattern of each gun u choose. Helps a lot when u spray and pray.
4. Dont spray and pray at range.
5. If u want to run and gun .. try to close the gap and always run zigzag or criss-cross fashion.
6. Learn how to grenade, helps a lot. Learn grenade spots in dust 2 as it is the map mostly in rotation. (left click --- throw , right click --- short toss) very helpful in case u want to flash fast.
7. Learn the map, playing on server with lot of people guarentee you will learn the pain points in the map (places where people generally hide). Check all the spots before u progress.
8. Listen to sounds, they will help a lot on predicting where the enemy is going and also help in pre-firing(esp if u have a smg)
9. Use the mini-map. It shows possible enemy locations and where your friends have died.
10. MOST IMP: Communicate.
Obviously, knowing how to shoot trumps everything but guess that is mandatory
.... also the age old army tradition of "Shoot first; ask questions later" works