There are a couple ways to Match with people:
If someone already Liked your profile, you'll see them in the Who Likes You tab. If you want to Match with them, tap the their profile and tap like and their profile will move to Matches, where you can start chatting.
You can also Like people who haven't seen your profile yet. Tap on Discover and tab Like (Red heart.)
Not interested in someone? Skipping to the next person indicates to us (and our algorithm) that you weren't feeling it. To skip in Discover, tap the X.