The Mastery screen (available under the Collection tab, after selecting a hero) will show your level progression with the selected hero. You can also see your upcoming rewards (slightly greyed out) and the rewards that you have already received, which will be highlighted in green with a check mark. Your approximate progression to the next level is also highlighted by the amber color in the progression bar, between the level numbers.
After you reach level 10 with a hero, you will be able to upgrade their Rank from 1 to 2 (then 2 to 3, and so on up to Rank 99). You can use Rep or Coins to purchase the Rank for your heroes. The amount of Rep or coins needed will vary based on the Rank of the hero. Ranks are only purchasable once every 10 levels.
If you are a new Paragon player, you can get Mastery Tokens by logging into the game, and will receive 3 within your first 7 days worth of login rewards. These tokens can be used to purchase the Rank 1 to 2 upgrade for any Starter Heroes. If you have a Mastery Token, you’ll be prompted to use it immediately.