Hello, welcome to the game.
When it comes to gaining power, as the post prior to me spoke about you'd want to check on the Power Up tab and check on just what you'd need work on. There's basic methods of gaining power that the game's tutorial already helps with you. That's being Equipment Upgrades, Ninja Tools, and Magatama.
Along the way as you level you'll unlock more features that'll increase your power. These are things like Eight Inner Gates, Summons, and Mysterious Cave.
Along with all of these, there are other, less spoken things that help you with power. Such as joining a group. Yes, being a part of a group is a huge benefit of strength. You're able to increase your stats using a Skill Tutor located inside your group's base. Various summons also augment your strength by quite a lot. Lastly, if I haven't missed anything. Doing TI and getting rare ninja tools greatly increase your strength as well.