Pip,

You clearly need to learn some basics. You need to know about intervals and how they are used to make scales and chords, which intervals sound good together, which don't. From there you can then start to learn about adding parts to a basic melody.

There are three ways to go about it. One is to see if a local college does a course in basic music theory. The second is to go to your local music shop and see what they have in the way of books on basic music theory. There are plenty of them, it's a matter of finding one which works for you.

Third you could try the internet. Google "Introductory music theory" or something similar and you will get plenty of hits.

This one looks promising. http://www.musictheory.net/lessons