Is is music or lyrics you're having trouble writing?
As for writing music, this is how I normally do it. I usually come up with a progression of base chords, for example a simple progression would be Em, C, G played in a loop. Then once I have discovered a progression of chords that I like, I start to mess around with them - break them up, play them in scales or arpeggios, play alternate versions of the chords, etc. A lot of writing music for me has to do with messing around until I discover a melody or progression I really like.
As for lyrics, they may start off as a string or words that I like. Then I might add to it a rhyming sentence, or another sentence that doesn't rhyme, or sing them to a melody that I like. As I said, for me personally a lot of writing music is just about messing around and building on things that I create until eventually they resemble a song.
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