I've been playin guitar for a few years now, have some knowledge and a decsent player. I have recently got a band together and started doing covers for the first few weeks. Now we want to start writing original material. My question is how do you know what notes/scales/chord progressions to use. I mean, do you use a major scale, or a pentatonic scale and do your chord progressions/ key coincide with this? Bascially, how do you write a song on guitar?
There are no rules when it comes to music, but general guidelines you could follow.
I don't know what your theory is like, but... first understand the major scale and how it's constructed, then read up on intervals... once you understand intervals you can understand how chords are contructed. Once you understand all that stuff you look into chord scales... that's where common progressions come from. Google it up and you'll get tons of results.
It really depends on the music you're playing, your influences and your overall creativity... like you never hear a metal artist for example just do a standard I-IV-V progression without adding some sort of twist... be it muted notes, little harmony or lick, harmonics, etc. that's what makes them who they are.
You can get books that are like "In the style of Mr.X"... helps you get a feel of how that artist constructs their rhythms and leads. These help quite a bit if you can't pick up their "musical patterns" (not physical patterns, but things they commonly do) on your own.