The melody usually interacts with the lyrics.
A marriage, in a way.
The melody should express and emphisis the lyrics.
Knowing scales and modes help. Not only in playing an instrument, but in songwriting and singing.
Practice, practice and rewriting always go along well with this. The more songs you write, the more experienced you'll become. You'll winnow the chaff and keep the good stuff that works for you. Everyone pretty much has their own system, or way of doing this.
If you have a good idea, and you play it, and it sounds like something you never intended, you need to rework it. Start from several directions.
Begin with the riff or lead. Ask yourself if it fits in with the melody. Does the lead expand and highlight the meaning of the lyrics?
In the end, all the parts of the song need to be seamlessly joined.
Stuff like that.
Sometime, take your favorite song by who ever it is, and write the words down, and the music.
Then, reverse engineer the song by taking it apart, dismantle it, piece by piece. Doing this may help in understanding where the lead and melody should meld. The lead is nothing more than a part of the song, without words.
Or as a guitarist will say, the lyrics are the part of the song without the lead.
Song writing is work. Don't kid yourself. You need tools, and scales, modes and a great imagination will help. Don't let yourself get uptight over some riff, words, lead part or a turn-around. When you get to the 'wall', put things away for another day.
Your mind becomes as cluttered as the memory on your computer. Give it a break.
Come back fresh.
Volumes have been written on this subject. It's pretty damn hard to cram some literary works into a forum.
Good luck.
