I need a guide to be a good chess player

So I started being interested in chess 2 years ago, but it only lasted for 3 or 4 months before I get busy in my study.

Actually, I'm a medical student who doesn't intend to play chess for a living, but I admire the game so much and I find it intellectually challenging, so I want to work more on my skills.

I'm a relatively good player, but not that good. So I just wonder if anyone could help me to improve my chess !! What should I work on first; Opening, Midgame or Endgame ? What books should I read ? What are the best and fastest methods to improve my chess skills ?

Thanks in advance :)

Have you checked out the study guides?  They're listed in the articles section under chess.com help.  The opinion of a working titled professional has to be worth something.....

Thanks a lot, " baddogno". I've checked this article section and it's awesome :)

Thank you too, "Estragon". Regarding the endgame, I'd really be happy to learn more about it, because in most of my games when it comes to endgame, I don't know what I'm supposed to do in this or that position. So after reading your comment, I've decided to read a book about endgame which is called "Basic Endgame Strategy: Queens and Rooks by Bill Robertie". Tell me what you think about this book. Would it be helpful or not ? and What are other books that may help me ?

In my humble opinion...

I'd recomend analyzing your own games, specially losses. Try to understand your oponent's plan, understand the options that were available to you that you may have missed. Using databases and engines can be helpfull, but don't forget to analyze on your own as well. This way should help you understand which part of the game  (strategy, tactics, opening play, endgames, whatever) you need to improvethe most and how your approach to the game can be refined.