How to write a guitar/piano part?

There is no wrong or right way to write music.

Are you trying to write music using scales(lead) or chords(rhythm)?

If you're writing chords, look at the scale degrees. Find the transitions you like. Each chord change sounds different. Find the degrees that have different 'feelings' with them.

I-V usually sounds happy, while I-vi sounds sad. I-vii(dim) sounds stressed out and depressed.

Keep track of what sounds like what.

As far as leads go, I recommend using arpeggios. While using scales allows for a wider creativity level, arpeggios are much easier to use to make a long string of flowing notes that fit together.

Again, find the transitions that sound like you want(happy, sad, angry, etc.), and play them as arpeggios, using varied pick sequences.

As far as rhythm... Rhythm beats me. I can write lots of melodies. I can't come up with rhythms to play them in.

If you have any questions about what I've said or if I didn't make much sense, send me an email(my profile has a link) and let me know.

You could if you know the general idea of guitars. Guitars have different notes and such but you could create some sort of beat in your head and go from there. Maybe do a little research like practice a bit on a guitar to get the basic idea. What about guitar solos? LOL Maybe a drum solo instead ;)

Every key has 8 chords that can blend in just about any pattern. For example; E, F#m, Abm, A, B, C#m, D, Ebdim. Just pick a few you like.