There is a lot of instructional material out there. So much so, that it can be difficult to separate the truly great info from the bad info. If I could answer this question I would be a millionaire. So, it is best to learn as much as you can so that you can get to the point where you are able to personally analyze and differentiate the "wheat from the chaff" so to speak.
Besides reading books from successful players (there are book references here at Cardschat), here are a few sites where I have found some great information:
Card Player Poker School - no longer being updated, but a lot of good material here
Poker Professor - this is really targeted for the beginner
Red Chip Training - some good free material here, but to see all of its content, there is a fee.
Splitsuit - some good free material here, but to see all of its content, there is a fee.
Advanced poker training - There is a poker training
App here and there are tournaments as well. Some good free material here, but to see all of its content, there is a fee.
Upswing Poker - some really good free material here, but to see all of its content, there is a fee.