CS:GO is one of those games that attracts a boatload of assholes. No matter how good or bad you are people will inevitably talk shit to you. You are not forced to play competitive at all but if you do choose to venture into competitive it will require you to level up to profile Rank 3 (A.K.A. "Private Rank 3") from playing casual matches before you have access to the competitive portion of CS:GO. This is due to people "smurfing" which is creating another steam account and buying CS:GO on it for the sole purpose of playing on an account that is lower ranked once you reach competitive so you can slack off and dominate lower level competition without having to really try at all. My main account is ranked DMG (Distinguished Master Guardian) and I also have a smurf that has access to competitive but I have not played any competitive matches on it yet.
There are two completely different ranking systems. One ranking system (profile rank) is in place solely for the purpose of forcing people smurfing into having it play hours of casual before they can ultimately reach their goal of smurfing in competitive matchmaking. The ranking system that people take note of is the competitive (matchmaking) ranking. Once you have reached level 3 in the profile ranking and are able to play competitive matchmaking then it will require you to win 10 matches before receiving your starting competitive rank. Below is a picture showing the competitive ranks and a link to the profile ranks:
Comp ranks:
Profile ranks:
http://counterstrike.wikia.com/wiki/Rank
Its perfectly possible to play the game without ever touching competitive and have fun. Sooner or later when you get good enough in casual gaming the game may become boring and that is usually when people then venture into competitive. The skins thing is mostly competitive swag. You're basically considered a smurf if you're ranked high or play well and don't have any skins, even cheap skins. I currently have a skin/item loadout worth somewhere in the neighborhood of $1100.
As for maintaining, if you do play competitive then it requires you to keep playing competitive at least sparingly because if you go a month without playing comp then you temporarily lose rank if I remember right.