In the inventory menu below your nickname (slightly to the right) you will see a green star, tap it to get access to all available bonuses and skills of your character.
The skills panel has three tabs: active, passive and temporary.
You can choose two skills out of the active skills list and assign them to buttons at the lower left and the lower right corners of the screen. Don’t forget that some active skills have cooldown.
Passive skills will be working from the moment you unlock them. Temporary buffs can be obtained with the help of pets or as a reward for completing a quest.