You have to play 10 placement matches before you are given a Rank. Your initial matches will pit you against a range of different MMR players to gauge where you belong; if you do really well by beating the higher tier opponents you'll get a higher placement. Don't worry about how the 10 matches turn out, you'll be placed as close as possible to where the system thinks you belong after them. Most new players fall into the mid-low Silver range, so once you get placed you'll probably face mostly Silvers.
Edit: Since the season is about to end I won't say "Don't play Ranked yet" (go ahead and get your rewards), but after you place you should focus on Normal Draft so that you can practice and improve so you climb when you do play Ranked. Don't hesitate to ask around for tips on improving, and watch some pro/high elo streams and read guides.