Lot's of advice here...
But do you play by ear, do you know theory, can you read music, what is your level, have you played in a band before, have you played live before?
First things I would do is know which version of the song they are expecting. But still listen to everything the original band made. Studio, live...
Then do I learn it by ear or by chart? Is the band doing it in the original key?
After deciding I would make a plan. What is the form of the song. Write it down. You'll remember it more easily if you write it down and understand where it is going rather then guessing...
Are there part of the song that are exactly the same?
When do they happen? Always at the same time or not?
Learn the first few sentence of that part to know where you are. Is it first, second or third.
Once I have figure it out I listen to the song. Making sure my plan is good.
Then learn the most important part. The fills should be last.
Say intro, verse x2, chorus, verse, chorus, bridge, chorus x2, ending. If you learn verses and chorus you have more then 75% learnt, then maybe the intro is a variation of the verse or chorus, it's one more down and so on.
Repeat for all of them.
Yes the first time is hard second time also but after a while it gets easy.
Don't give up!