Alright, time to clear up some misconceptions:
1. CS:GO is a competitive team game in which you are given money to buy weapons every round (Maximum of 30 rounds to be played in a match, with each round being on average 2 minutes). If your team wins a round, you get $3.2k. If your team loses, you get a starting amount of $1.4k. Should your team loses consecutive rounds, I believe you get an extra $500 added onto the base amount and subsequently amounts.
If you were to lose your guns due to death, you'd have to buy again.
2. Yes, you can loot guns from your fallen opponents. However, you cannot sell them. You can pass looted guns to team mates who have little money to make a full buy. (Full buy means to be able to make a purchase of a primary weapon, kevlar + helmet and grenades/flashbangs/smokes.
3. You can obtain weapon skins by either unlocking operation crates, or continually playing matches (both casual or ranked), or buying them from other players via the Community Market on Steam.
4. You'd have to reach Rank 3 in order to play Competitive mode. So that means you must play in Casual, or any other game modes to level up to Rank 3. This is a good thing, because this means you get to understand the mechanics of the game and how the recoil works in the game. In time, you should be able to grasp the game and be alright in it. Also, flashbangs and smoke grenades play a vital role in securing/defending bombsites.
Also, allow me to reiterate myself: CS:GO is a team game so your team might make out calls. There are different calls in the game for both Terrorists and Counter-Terrorists.
An example would be in the Dust 2 map. If you are playing on Terrorist side, one of your team mates might call out 'Let's rush b'. What this means is that your team would be pushing to the B bombsite without stopping and have the bomb planted.
5. Not entirely sure about this one.
EDIT: I suggest that you watch a video of competitive play on Youtube. For one, you might want to check this video out. It is a Fnatic vs. NiP (Both prominent teams in the CS:GO competitive scene)
https://www.youtube.com/watch?v=iSqxzRtwi80. Analyse how they play, how they take over/defend bombsites and what they buy.