Keep aimer at head height
When in 1v1's you need to strafe from left to right, sometime more advanced would be circling around them.
When shooting somebody at long range it's best to crouch because this reduces your bullet spread by a lot, so therefore you will be more accurate at that distance. However if they're shooting at you, don't just stay still and crouch because you are just going to get hit.
Always move around, get used to the keyboard and its buttons so you can move freely while pressing other buttons and not have to stop to look and move your hand.
Little finger on sprint (shift), then one on each A W & D, with your thumb on jump (spacebar). You can easily just move your finger next to your thumb aside to reach reload (R) or activate (F).
Keep your gun reloaded always.
Turn auto reload off so you can switch quickly to pistols when your clip is empty.
Since you're not too great at close up fighting, use mines and artillery for some easier xp.
When planting mines, think about where you would walk and where other people go. Don't just put mines in the usual places, put them where people won't expect. Think about it, when people run around corners, they don't go on the outside of the corner they try and cut it a bit on the inside to make their trip quicker. You can't jump around corners and therefore a corner would be an unexpected place to put a mine.
Hope it helps.
billy