The technical (non-artistic) side:
Well, write a sentence, any sentence.
Now write a second one, with the same amount of syllables.
Now write a third one, which must rhyme with the ending of the first line.
And a fourth one, which must rhyme with the ending of the second line.
Now read that loud rhythmically again and again. Maybe write a few more verses. Try to hum it. Anything yet?
I started by changing the words of existing songs, usually making parodies like Weird Al. I still do it for fun sometimes.
Here’s a random example for you:
Let It Be (Beatles)
When I find myself in New York City
I can find nowhere to pee
All the booths are taken, let me pee
And in my pain and anger
I just pee upon a tree
I can hold it no more, let me pee
(yes, stupid, but it rhymes and I just made it up in a few seconds).
Now, for something more original (also making it up on the spot) :
I am sitting here at home
Writing lyrics just for fun
And I’m bored and all alone
And I’m waiting for the sun
Again, stupid and useless, and I’m not going to keep it or record it, but it rhymes. Now if I could change the words to have a slightly more meaningful message to convey, but keep the syllables right, I’d have something to work with. Maybe I could change the last line to “I am waiting for my mum” and then write a song about my mother. Or “I think it is time to run” and write a song about escaping. And that’s where inspiration comes in.