After completing the tutorials, you can continue playing on Practice Mode vs the bot until you reach Level 15. This will allow you to gain the XP you need in order to level up and begin unlocking the different prisms and heroes.

Once you reach Level 15, you will no longer earn XP on Practice matches; however, you'll then be able to play Ranked with other players and continue earning XP there.

Ranked

You can play Ranked - Constructed mode when you reach Wanderer rank.

Discovery

You can play Ranked - Discovery when you reach Level 35.

Conquest

You can play Conquest mode when you reach Trainee rank.