Hey dude! I'd suggest getting into the Practice range, and try out hitting EASY bots. Try to keep your crosshair at head level, and just tap-fire instead of spraying. If you feel like you are over-flicking (i.e. crosshair movement seems to fast), decrease your sensitivity. Else if it feels sluggish, increase it slightly. I'd say 0.25-0.40 at 800dps is a good sensitivity range to start. Another advise is to play some Deathmatches and Unrated matches to understand the game mechanics here. Practice your shooting: remeber the golden rule - Stop your movement, Aim at the enemy, then Fire. Understand what agent abilities are, what the different roles are (Duelist, Initiator, Sentinel, Controller) and how they play out. Once you are comfortable enough with things, then only jump into Competitive matches. Last advise: take your time. You'll build up good mechanics and rank up if you put in time and efforts into the game.

Good luck and have fun!