I need to learn how to read sheet music because I want to join jazz band next year, how should I go about learning it?
I would check out what the band expects. For jazz, lead sheets (aka “fake sheets” or “jazz charts”) are more common than full scores.
You may want to check Justin’s Jazz series, especially the parts about Fake Books, Jazz Standards, and Jazz Charts.
I recommend going through William Leavitt’s A Modern Method for Guitar. One of its goals is to teach you to read standard notation. It’s a full guitar method, so it teaches other stuff, too, but if you go through it you’ll definitely learn to read.
FWIW, when I first started sitting in with jazz guys, this is the exact book(s) that they gave me to learn to read. (Actually, I already knew how to read standard notation since piano was my first instrument, so I could read, but I didn’t know how to apply that to the guitar’s fretboard.)
Majik is correct that jazz usually tends to rely on lead sheets rather than full scores. So you’ll have the harmony notated as chord names (e.g. G-7, Cmaj7, et cetera). As far as reading standard notation goes, you’ll mainly be reading the “head” (melody) in treble clef. And improvising a lot…