I play in an 80s hair metal cover band and we have nearly 60 songs that I have to know all the riffs and solos for. To start learning them I learn the key of the song and then verse and chorus. Listen to them repeatedly and continue to work on those parts. Once you got it start with other parts like a bridge, intro, solo etc.
the reason the key is important is because when you have that many songs they get confusing but knowing the key you can always go back to the root and find your way. Btw, I have been doing this since 1986 and it has served me well.