You need to get your gear to at least hopebringer to be able to level them past 10. You get plans from the heroic realm, and materials from auto-level and quests.
Getting runes at rune temple could power you up a bit.
You get more slots to put heroes in from Royal challenge.

You don't need to spend any money to progress in the game. Spending money will only get you there faster.

Hope this helps.