Ok so im trying to write a song. I have an intro riff but how do I know what key it is in to write the rhythm for the rest of the song (during the verse its going to be mainly bass and drums and vox but I wanna put in some chords for rhythm). And then how do I know what scale/box to use for the solo?
"Imagine all the people, sharing all the world"
-John Lennon
well, from what i know, since i'm no theory buff yet, i'm working on it, count the accidentals(sharps and flats) in the song. like if you have an fshapr and then a c sharp, and that's it, then that's probably in the key of D, i think that's the key, but just count the accidentals, and from that, find what key has those accidentals and there you go. I myself have not really gotten a grasp of how to use box's and such to write solos, hopefully someone else can help you.:]
Well, you could modulate and change keys with a chord that will halp you do that (diminished maybe), the intro can be its own little thing. If you want it all in the same key, look at the sharps/flats like nautaflcl said, use the circle of fiths (google) to determine the key. Major/minor is really just dependent on the chors you use.

Quote by AndyGray
If you go on google and type in 'all guitar chords'. You get a site called www.all-guitar-chords.com (name-ish) Go onto reverse scales and map the notes out from the song, If there is enough to be sure what scale it is, it will tell you.

That might help too, can't hurt.