do you want cyclopentadiene, or a substituted form? you can make unsubstituted cyclopentadiene by cracking dicyclopentadiene in a retro-Diels-Alder reaction.
Substituted cyclopentadienes are a little harder, but there are a few ways depending on what level you're at. Simple approaches (on paper!) include grignard addition to cyclopentenone then elimination; more complex approaches might rely on a catalytic cyclisation of alkenyl-allenes with a something like gold(I)phosphine complexes.