There are three ingredients to improve at chess and more often than not, chess players do 1 or at most 2 of these.
They are Observe, Learn and Practice.
A player needs to observe for themselves. Without observation its very difficult to learn. One needs to look at the games of the greats, his fellow players. One needs to look at their own games and see how they hold up against the classics. Without this introspection its very hard to know where to put your attention on.
Once you have done your observation you need to sit down and learn various aspects of chess. Chess knowledge is vast and a lot of people go about this process the wrong way. The first thing about this part is that not all knowledge is the same value. Yet players just grab random books and start anywhere and hope to get amazing results. It simply does not work like that. More general books help at the beginning with building a base of understanding and as you get stronger you become more specific..
Lastly all is useless if you do not drill the information you are studying. Having learned you need to drill and exercise that work until its second nature, either with you peers in the club or computers if you don't have friends or club.