what is the best way to improve my game?

I'm 18 and I've been playing poker on and off for about 2 years but in the past few months I've started taking it

seriously and I want to try to become profitable enough to make an ok side income while I'm at university. At the moment

I'm profitably playing $1.50 SnGs but I feel that there are so many situations where I could play much better and I want

to learn how. What would you say the best way of improving my game would be? Books, coaching videos, play more? By the

way I can't really afford a coaching site so any online resources would have to be free, I would be willing to invest in

a book though. Thanks for any advice