basically i have been playing piano for quite some time, but i have always had to rely on people posting video's on youtube/toiling over countless pages of sheet music.
I want to know how to train my ear, so that i can listen to a song, and then just be able to play it like i know many people do.
Your ear will train itself. If you study music, composition, harmony, etc. then you will find that after a while it becomes instinctive. Practice a lot! There are no shortcuts. If there were, then everyone would be an amazing musician. Best of luck.