To summon new awesome Heroes, go to the Summoning Portal in the middle of the Town screen.
Troubleshooting:
If you can't use scrolls in the Summoning Portal (buttons are grayed out but you do own scrolls) it means that you don't have enough space in your Barracks to hold new Heroes.
Retire some unused or unwanted duplicates, or expand your Hero capacity by purchasing more capacity in the Shop.