Well, if you want to work out a melody to sing, using the notes of the other instruments....then I think I can help you.
Say the guitar is playing the chord C, right? As the basic C chord consists of the notes C, E and G, you can use those notes to make up your melody.
Say for example that the guitar plays the chord of C for 8 beats....you could sing C, G, G, E, C, E, C, G -and there's your melody which will go perfectly with the music!!
Once you can make a melody from the basic notes of the chord, you can start progressing, and making the melody more intresting. This is where arpeggio's and scales come in.
Take again the arpeggio of C major. (imagine your guitarist is still playing the C chord). The notes of the C major arpeggio are: C, E, G, C, G, E, C - these notes could form the basis of you melody too.
You will need to adapt the notes of your melody as the notes in the music changes....take this as a basic piece of music. The guitar is playing the chords on the top line, and you are singing the melody on the bottom line:
C D F G A C
C G E F A D F C A G D B A A E C G C
are there you have it - there's a basic melody, using the same notes as in the chords in the music!
It's also good to just have a play around. if you know the words, get the musicians to play the music, and try things out -you'll find how your voice will probably automatically find notes which suit the music after a little practise!
Good luck, hope I was some help
Love Laura
XxX