Theory and Math
Learn and apply the mathematics of poker. Without it, you are playing from the gut and that will not cut it in the long-run. How do you learn? You pick books, videos, forums, or other strategies that suit your learning style. Then you study, study, study, and study some more.
In today's world, expertise is taking a back seat to personal opinion. People mistakenly believe that their opinion is just as valuable as an expert's. Hogwash. A person is an expert in their field because they have studied and lived in that world. When a blowhard dismisses expertise and claims that "this is what I think" without any supporting theory or background knowledge they are destined to lose. So it is with poker. We can all have our 'opinions' on what is the best play in a given situation, but the math of the situation determines the optimal action.
I am a book learner. So I have a library of books that focus on my preferred game.
Decide how you want to learn then buckle down and go for it. Good Luck.