If you have something to add, disagree or discuss, feel free to express yourself in this post.
I'll add in this post (or edit this one) a bit later some strategical tips which might help you.
I struggled much before putting it in Tutorial section and not in General section. It's not a tutorial. Just wanted to share my views on improving ones abilities with you.
And english isn't my major so sorry for my mistakes and misused words.

I know that the majority of you know a lot of it...............but still....

Don't be scared.
Hesitation is the big enemy, any action is better then inaction in many situations.

Don't panic.
Try not to panic as much as you can. If you try to stay calm, stay cool and keep you relaxed, you will do a lot better than if you just panic all the time.

Expect the unexpected.
The enemies like to be original.

Camp.
A lot of people really hate that, though it is a wonderful strategy which allows you too take down enemies with least damage to yourself.
But beware. It's effective to do so to hold the enemies or to take down an approaching one. But it's completely uneffective to camp because you're too scared to do anything else.
For example, at de_dust2 if your whole team started to push A and no one even bothered going to tunnel, it's OK to stay behind at spawn to hold the rear of your teammates.
But it's not OK to sit the whole round watching wooden doors with 5-6 of your teammates doing the same. Try to be effective with your camping.

Isolate the enemies one by one and expose yourself to a minimum.
If there's several enemies VS you, you wanna cut off the angles so you have to deal with one enemy at a time. Cause a second you pop out too far and get seen by more than one enemy, you can only get one kill max. The second guy (If he's not an idiot or some weakling) will kill you. So expose yourself to least ammount of enemies and it will get you the most kills, be effective and will keep you alive.

If your teammate near you died and you're not sure where the enemy was,
play passive
and let the enemy come to you. Or, at least, wait for your teammate(s) (to finish the job) before playing a hero.

Do not immediatley reload after initial frag due to possibility of more enemies.
The enemies might come out trying to catch you reloading.

Use damage noises to expose enemy players.
When someone's body is hit it makes a certain "damage" sound.
If you spam a wall and hear it, then spray your ass off trying to feed this guy with maximum bullets possible.
And you dont need to check behind every box. Just spray a little at where the playes usually sit.
If you cannot hear anything means it's clear.

Need to spam a wall?
Spam 20, save 10.
You never know when someone is gonna pop out.

HE Nades
DON'T
-Frag opponents
DO
-Control space
-Push enemies back
-Weaken enemies

Try to be effective.