Um the best thing to do is to sing it the way you plan on having it sung then, try to figure out what notes by playing by matching them to notes on the guitar from there, pick the main ones and turn them into chords
how long have you been playing bass/guitar?

best thing to do either way is to learn some music in the particular genre you are going for. this way you can get a feel for what is expected of your song.

then you need to completely forget all of the specifics of all of those songs that you learned. take the technics and just that.

i play for a screamo, hardcore, metal band...but when i am at home i practice Lamb of God and tons of heavy hard stuff.

the more you practice, the easier writing will become

Well is it a happy song? sad song? What genre?
Quote by cakemonster91

*chuckle* A peanut. With a face.

Go to your staff paper and re-write this song a half step down so on the paper it'll be like you have a "C" just move it down to a "B#"

Know your theory, then play like you don't.

Sing, and then chose a progression. A likely one would be I-IV-V In A this would be A(I) D(IV) E(V) Another good rock/pop punk progression is I-V-VI-IV