You’re bound to make a few false starts, and you’ll soon realise that your choice of radif is the single most important factor in determining how successful your ghazal is likely to be.
Another traditional topic is love (where it’s often deliberately ambiguous whether the poet is referring to divine love or love between two people). At the same time, however, the key word is “traditional”. As with any poetic form, you can write about any topic that works. See also for example Patricia Smith’s “Hip-Hop Ghazal”.
Remember, each couplet constitutes a separate little poem, so don’t have one verse rely on a previous one to make sense.
Traditionally, the poet’s pen-name was included in the last verse; this final couplet usually contains a ’turn’, or change of tone, to something more personal or quirky.