to start with i would honestly play casuals such as team death match arms race etc. then once you feel you are ready you can play competitive vs others.

in competitive it's 5v5 vs people of roughly your same skill group. i'll assume you know the basics of how to play terrorists and how to play counter terrorists.

When playing in casuals practice trying to work on your aim and learn what recoil patterns each gun has.

you can alternatively go on to the steam community page and look at aim assisting/teaching maps to help improve

each week you can be dropped 3 cases and 3 skins(it's random and sometimes you don't get any depends how much you play) this resets every thursday. you can buy weapon cases from the steam market and keys(the keys cost 1.59 gbp each not sure about other currency) but you aren't guaranteed a certain skin just once randomly from the list of skins that particular case has. you can buy skins and knives from the marketplace these range from about 0.03-400+

this was a basic and pretty quickly made comment so if you have any questions feel free to reply to me and i will do my best to answer them. don't forget to practice and do your research!!!