How do you stay on topic

[busker]

It's an unconscious competence. Like anything, if you practise enough, it'll come naturally.

I don't bother with end rhymes. There are so many half rhymes and slant rhymes in normal speech that end rhymes are unnecessary. The thing to strive for is beauty.

More practically speaking, if you're starting out, try experimenting with strict verse forms in the Poetry Practice section. Once you're able to write a sonnet on whim, you can move on to greater things.