How to improve as a poker player

Depends what your current knowledge level is.

But for starters.
-Opening ranges and the why and how behind them.
- hand classifications (premium, marginal, draw, junk) and how to play them on wich board.

=> I found the basic course from Jonathan Littles pokercoaching pretty good for poeple who are trying to become a better player. (And it's free if I remember correctly)

Once you are playing a decent amount , I think it's very important to review hands to improve. (Especially the why's behind every action)
Wich as a amateur player with limited free time isn't always a fun way to spend your time because you would rather be playing. But it's the best way to improve i.m.o.

And posting your hands here is a great way to start.