Before I begin, I just wanted to explain that I really want you guys to get better. It would make me stronger if everyone was an IM or something because I would learn from you guys. I'm not doing this for profit, just out of my dedication to chess and to all of those people who are struggling to improve.
Note: This is intended mainly for people with a rating of around 600 - 1400 in live standard or 1000 - 1500 online.
But anyone can read this and improve.
How to get better:
The answer is simple: long games. Standard or any online works fine. Be sure to go over your games in detail after they are over.
You can still play blitz or bullet for fun if you have time.
The first thing you need to do is learn tactics, and then get better at them.
Also do your 3 problems a day (if you are basic) or do as many as you want (premium).
Make sure to do tactics. Without tactics you can't do anything.
This one goes 10 moves deep if you don't pay money, and unlimited if you pay money.
GM games. Free, unlimited moves. The only drawback is that there are relatively few games here.
The most useful of the 3. Goes unlimited moves deep and has a LOT of games.
Gives the true value for any position with 6 men or less.
Middlegames: Just play more games and you'll naturally get better at these.
Remember these things to consider in middlegames, and probably in all phases of the game:
Tactic - Can I win immediately with a tactic?
Plan - What is my plan?
Candidate Moves - What are some attractive-looking moves that I might consider playing?
Pawn Structure - What is the Pawn Structure of the position?
Threats - What are my threats, and, more importantly, my opponent's threats?
Here's a really helpful website in which a NM gives his advice in free video lessons.
Hope this helped. Have fun fellow chess players! :D