competitive is the best bet
warmup-play deathmatch-play wingman 3 times or so - then keep playing comp with friends if it's possible.
First take your keyboard and smash and take your mouse and throw it from the window and then try to play, you will get good aim XD
by playing competitive , and win the matches
Play deathmatches if you want
Is a very good game but i dont have many tips
It depends on the rank you're referring to. You said you were rank 5, so I think you're talking about level. The quickest way to level up is to play and do well in competitive matches. Get kills, win games, plant/defuse bombs. You also get weekly xp, so if you play a couple times a week you won't have to suffer from the xp reduction after playing a lot in 1 week.
Competitive gives 120xp per won round, but since you're a new player it's really better to stick to Casual, and Deathmatch if your aim is getting better with some weapons.
I would say stick to casual until you have a good feel for what you're doing. You will rank up slow but don't worry about it too much. If you start playing competitve as soon as you're able to, people can be mean :)
you can't. recently this game has become complete rng bs
Be topfragger and you will rank up
Deathmatches. If you mean RANK up, not LEVEL up then... I can't help you, I'm trying myself too.