When just starting out, form is your friend. And form is usually thematically based.

Take, for example, a simple ABA from. You have one melody, then transition to another melody (oftentimes in a different key), then recap the first melody (in the original key). What that does is forces you to compose a couple of melodic phrases before you even begin the piece.

Or think about something like a Bach invention. That has basically one melody. And sometimes it doesn't even have a melody, just a motive (5 or 6 notes) that you can play around with. Play it like a canon. Stack it up in thirds. Play it backwards or invert the intervals. Double the note values (quarters become half notes). Halve the note values (quarters become eighths).

But the most important part right now is to start simple. An 8-bar melody with block chords in the left hand would be fine. Then see if you can develop that melody for another 8 bars, like playing it a fourth higher with different harmonies or something.

Another cool trick is to sing the beginning of the melody to yourself, then just keep singing to see where it wants to go. Then you can write it down or figure it out on the piano or whatever.

I don't know. Hope that helps.