I need help with Composition!

I have a mock GCSE exam in School in Music.

There are 3 parts, Solo performance,(I'm gonna do Paranoid by Sabbath :D)Group Performance(Beat It by Fall Out Boy suckish huh?)and Compostition.

I really wanna get an A* or A in all, and Compostition is the hardest, so I need some songwriting tips, I am going to write Guitar(Obviously),Drums and maybe Vocals.
The thing I really need help with is mixing them together, that's the paart I need help with.

Any help is much appreciated!