Hey there! :)

So.. I'm at a point where I want to write out some of the music I have been composing. I could just use tablature since the majority of the parts could simply be played on guitar and bass.. but I wanted to be able to write for my church band. There are two band structures I need to deal with:

Small Regular Band:
Piano
Guitar
Female Singer
Male Singer
Congo Drum (not all the time.. and he usually just makes it up as he goes along)

Bigger Band:
Keyboard
Guitar 1
Guitar 2
Bass
Drum (full kit)
Female Singer (sometimes 2)
Male Singer (usually only 1)
(Occasionally there will be another instrument.. such as a flute or a saxophone but this is an exception to the rule. The extra instrument situation occurs even less frequently than the Congo drums situation from above)

I've been reading Music Theory books, including how to write sheet music but I am a little overwhelmed still with what I need to do. I want to be able to compose almost everything on the guitar if I can, and use the keyboard as little as possible. I can easily create the key, tempo, and time signatures. Create chord progressions, melodies, bass lines, counterpoint and harmony melodies all on the guitar.. but the bit I am having trouble getting my head around is where to begin writing out the sheet music.

I don't even know where on my guitar is the middle pitch. I'm assuming that I need to treat the guitar in a similar way to the piano. The piano has the middle C which you can see below right in the middle of the treble-clef and the bass-cleff. This makes it extremely easy to know where to begin writing down the notes on the staff. Now this book I have says that guitars are an octave lower (or higher.. I can't remember off the top of my head right this second) than concert pitch.. but where on the guitar do I start and finish on the staff.

If I play a middle C on the piano, then I play that "pitch" on the guitar.. I can play it in at least 2 different places, which makes it even more confusing and frustrating. Is there some way, like the circle of 5ths, to immediately know which particular "pitch" fits into which particular position on the staff?

I think that is the key point of it all, is that I need to think about it in "pitch" rather than notes of a particular octave. I can play a note of a particular octave at least 2 places on the guitar (except for the low and high end parts of the guitar's vocal range).. so I need to know the pitch rather than the note, and how that fits in with the concert pitch. I feel I am very close to understanding this, so may be I just need someone to say it all again in a different way with some examples on the guitar.