Once you receive your first ring from the game, it will automatically unlock the Skill tree feature for you. You can find the Skill tree from the Loadout by clicking on the Info & Upgrade button.
Each skill requires rings and coins to be unlocked. The higher level skills also require your character to be Starred up. You can see the Star requirement on the top of the Skill tree.
The Skill tree has different paths you can take, so make sure to check out both of the skills in each path before unlocking a skill. If you think you made the incorrect choice with your path, don't worry. You can unlock all of the skills, or you can use the Reset button on the top to reset the Skill tree, and return all the used rings to your account.
Different characters have different Skill trees. Each Skill tree has one new main skill that you upgrade throughout the whole tree, and different buffs to your character.
You can find the Star Up feature from the Loadout by clicking on the Info & Upgrade button, and then choosing Stars from the top right. To Star Up your character you will need Hero Shards.
How to get rings?
Rings drop from the enemies in Elemental Castles, from the Summon Gates and Summon Banners. Once you have 5-starred a hero, all of their shards are automatically converted into 5 rings.
How to get Hero Shards or Super Shards?
Hero Shards drop from the enemies in Elemental Castles, and from Summon Gates. The Daily Events also have a chance to drop a shard. During a Hero Special Event the Hero Specials will have a higher chance for a specific Hero and weapon shards, make sure to check those out!
Once you have 5-starred a hero, all of their shards are automatically converted into 5 rings. On top of that, each conversion has a chance to create a Super Shard for the hero. Super Shards can be used to star up the hero from 5 to 10 stars.
Stars above 5 introduce the ability to roll an improved version of your heroes' unique perk during runs.