The way it works is that you select your map or general game mode, as in Defuse Mission or Dust II if you want to be specific, and join a queue. Not too long after, it'll ask you to "ACCEPT" a match. You do not know who's playing unless you're in a party with someone. Afterwards, you start the match with a 1 minute long warmup, in which you have 16000 dollars to spend however you want. Then, when the match starts, you start with only $800 dollars. How to get more? Get kills and win.
If someone leaves during a match, a bot takes their place for the remainder of it. However, you will be penalized in terms of your rank and how long you have to wait until joining another match if you leave early.