To unlock more tracks, cars and characters, you need to level up.  

Every time you upgrade your Powerups, and win races, you earn experience points.  If you earn enough experience points, you will increase your level.  You can see your progress in the upper left hand corner, next to an icon which indicates your current level.  The fastest way to level up is to upgrade your Powerups.

To upgrade your Powerups, you need to win and open chests, which will give you Powerup cards.  Once you have enough cards your Powerup will be ready to upgrade.  Upgrading Powerups makes them more effective against other players.

Each time you level up, your cars all get faster, and you unlock new tracks, new challenges, and new Powerups to discover in the treasure crates.