Basically, when you start Ranked Mode, you will fight 3 CPU opponents, then 5 players. Given that Bronze is the lowest league, you theoretically could lose or draw on all matches, and still get in. However, if you do well and win, you could easily start in one of the higher leagues. I got silver with 3 wins, and the bronze achievement still popped, so it looks like everything is stackable. In order to actually rank up, you have to win. You get bonus medals for doing so, which translate into lightning coins. You can lose rankings as well, so be prepared for a grind.
WARNING: This game's servers are terrible, and you will get disconnected plenty of times. These almost always count as losses, so beware. However, if the opponent leaves, it will count as a win for you