well, that's what we're all here for - to learn.
We can't just tell you how to write music. What we can do is give you specific advice for elements of music composition that you can work with. The first thing I would suggest is getting a book of theory or even just going online to some basic theory sites so you can understand how a chord works and how melodies go over them. Probably some of the conservatives here will tell you to get a teacher or a book, but really, if you just want to know a bit about chords to write pop music, try
Ricci Adams' Musictheory.net, or
teorķa - Music Theory Web - Espacio dedicado a la teorķa musical (it's in English too). Just knowing how to play piano will not necessarily make you a composer, remember.
Alternatively, just get MySong, sing your melody and it makes an accompaniment for you.
Of course, I would dislike you very much for taking that route.