ok so Ive wrote a couple song lyrics and Ive got some guitar parts for them, but my question is, how would I write parts for the different instruments? my brother plays bass and he wants to start playing my songs with me but I dont know how to transfet them to bass, so how would I go about doing that? thanks for any help and I think this is the right fourm...
Write out the lyrics with the chords and play your brother the song. As a bassist, he will know where to start, and then you can direct him.
For the bass, you could simply double the guitar part an octave lower, however this isn't using the instrument to its full potential. You could also just write another part for the bass that compliments what you are playing, it shouldn't be too difficult to write, considering the average bass is simply the four lowest strings of a guitar tuned an octave lower.

If you want to write for drums as well, then you should familiarize yourself with the names of the parts of a contemporary drum kit (know what a snare, hi-hat, etc. is) and familiarize yourself with what sounds these create. With that knowledge should be able to communicate the drum part of your song effectively to a drummer. Wikipedia has isolated sound clips of various drum kit components if you want to familiarize yourself with them. http://en.wikipedia.org/wiki/Drum_kit
You'd basically just follow the chord tones with a bass line. You can keep it simple by just playing the root note of each chord, or you can try and get a bit of a melody going that utilizes the thirds and fifths of the chords as well. Since the guitar and bass are so similar, you can basically just look at the chords you're playing and pick the notes right out of those shapes, and your brother will be able to play the same thing.