[Stephen]

I agree that it helps to learn songs and study their construction as you begin to write your own. On the other hand, if you feel the urge to create, there are some hints that will help you as a beginner.

First, not all the melodic possibilities of simple chord progressions have yet been discovered. New songs with a basic four-chord pattern such as C Am F G or Am G F E, repeated over and over, are still being written. Try a basic progression using using chords that you know will work together (C F G is the simplest, but you can change the order and stay on any chord as long as you like). This will allow you to compose new melodies by listening to the notes of the chords you're playing but will not tax your brain trying to find new progressions by throwing chords together and seeing what works (though this is a valid technique to try later when you understand more about harmony). Meanwhile, try taking a song you know and writing a new melody for it using the same chords.

Second, if you have a flair for writing, it's not too hard to come up with some lyrics that rhyme. Write a new lyric for that melody you just wrote and you have a new song. Don't let your lack of experience stop you. Think of something you want to say, write about your experience or about what you know. If you don't like the results at first, that's normal. I still start about 3 or 4 songs for every one that turns out well. The others are scrapped. As you learn more about songs, chords and progressions, the whole process will get easier.