New players will only have a few Heroes available at the start to help with learning game mechanics; and unlock more Heroes as they continue to level up their account, unlocking all heroes at account level 9.
New heroes are released often and will be free for all players. You can try them out by locating the Solo vs. AI option in the Play menu, above the Start button. Doing so will place you in a private match against AI-controlled enemies, as well as populating your team with AI controlled bots.
In Solo vs. AI matches, you will gain full XP and Reputation up to account level 15. After that, Solo vs. AI matches grant no XP or Reputation. Additionally, you can leave a Solo vs. AI game at any time, without receiving any penalties.