You have a beautiful accompaniment and maybe you have words. So how do you go about creating a tuneful vocal part that pulls it all together?
Figure out the key of the accompaniment by singing a scale that goes with the chord. Look in the music book for the ending note of the accompaniment in the bass (the lowest note), that will usually be the key you are in.
Sing or play that scale to yourself. Do,Re,Mi,Fa,Sol,La,Ti,Do. If you have trouble with this step, get someone to sing a scale over the piece of music for you. The scale that the piece of music is written in tells you what notes may be included in your vocal melody. These are the notes you will use to create your vocal melody.
Sing or play the scale over the piece of music. Notice that some of the notes fit certain places better than others. Remember what you like and where. This is the beginning of your melody.
Listen for parts that sound nice to you-- where the notes you are choosing sound good with the piece of music. Record these places (or write them down if you read music).
Tackle decreasingly smaller portions of the piece of music, until you have a general outline of how you want the melody to go.
Note that when you have lyrics, say them over and over and decide roughly where each line should go. Fit each line of the lyric to the appropriate piece of melody. You may need to add notes (sing the same note twice or sing a note right next to the note you had in your original melody).
To fit the lyric to the music, look for the repeated portions of each. The first line of the first stanza of your lyric should fit the music the same way as the first line of the second stanza of your lyric.
Remember to write from the heart. Nothing is better than what you've been through and what is going on around you.
Some parts of the melody (like high notes, or notes held for a long time) will draw attention to themselves. Make sure these parts are attached to words in the lyric that you wish to emphasize.