Hi and welcome to chess.com 
In live chess, the computer typically assigns you a human opponent based on your ratings (it tries to give you someone of similar rating/ability unless you change the deviation in the settings, which I recommend just leaving alone). However, this is difficult to do as the computer takes a while to "figure out" your true rating/chess ability. When you play more chess games, your game sample size will grow and the computer will be able to more accurately give you closely rated opponents more often.
Also, don't worry about rating - especially when starting out. Under most games, you will gain 8 rating points for a win, drop 8 for a loss and change nothing upon a draw. However, when you start out the computer will swing rating erratically-looking to the untrained eye. You may gain several 100 points for winning a single game, but then lose a ton of points upon just one loss; do not worry, this is just the computer trying to stabilize your rating efficiently to what rating range you probably belong in. After several games, maybe 30 or so, then your rating will swing less upon results and you will likely be paired against opponents more similar in ability to you.
(If you like looking things up, you can research this; chess.com uses "glicko" rating system [very similar to "elo" rating system with a slightly different "k-factor" in the calculation process]).
The good news: the more games you play, then the more accurate the computer can pair you against someone closely rated. Just play a lot of games and have fun
I just created my chess.com and I'm wondering when I first start playing PvP matches, will I be matched with any rank or can I set my level to something around 900 at first?