The best and easiest way to get free skins in CS:GO is to have the Prime status upgrade to your account. Surely, you have to pay for it first, but it will multiply your investments indefinitely as long as you play, play and play! Prime's profile levelling system is what gives you free skins.

Prime account holders get skins and graffiti drops from leveling up their profile in addition to the random drops at the end of matches. Those without a Prime account only get drops at the end of matches. Graffiti drops are pretty low value, so you want to focus on skins or cases.