How to become a pro in three minutes:
lower your sensitivity. Most pros are under 1600dpi (a lot are like 400dpi 2 ingame which is the equivalent of 800), while I use 1600dpi everywhere (1600dpi 1 ingame). They recommend you being able to make ONE FULL TURN on your mouse pad by bringing the mouse all the way across it. With a high sensitivity, headshots are fairly difficult. Change it now so you learn with a lower speed instead of a super high speed and then you decide to try out a lower sensitivity and mess yourself up for weeks. Also turn off mouse acceleration and use raw input.
https://docs.google.com/spreadsheets/d/12PSHqb8Vwg8rSCOkGjbbsj8iBsm8p52jOLffDc88iy8/edit#gid=0
You can change your crosshair so that it displays when you are inaccurate, these are dynamic crosshairs. There's tons of sites that help you make crosshairs, just search crosshair generator. There's even maps to do it.
http://tools.dathost.net/ I strongly recommend having a dot in the center that doesn't move.
Learn flashes and smokes and when nobody uses them at lower ranks you will dominate them.
Practice your aim a lot and you will dominate in lower ranks. Deathmatch and the aim_botz map are pretty decent ways to learn
There's tons of more stuff to learn, especially when to buy and what to buy, but i'm not sure how much sense that stuff will make until you've played a couple of competitive matches. /r/globaloffensive has a bunch of guides and stuff on the wiki you can use, at lower ranks if you have decent aim and decent tactics you will dominate frequently. Aim is supposed to be the most important parts until you get to much higher ranks, if you can aim well you will do good. Knowing angles and stuff really help as well. Also, you will probably get rekt your first few competitive games since you start out at the middle of the elo instead of the bottom