General:
* Elle est drôle. (Most common and literal translation)
* Elle a de l'humour. (She has humor)
More specific:
* Elle est très amusante. (She is very amusing)
* Elle est pleine d'esprit. (She is witty)
* Elle fait beaucoup rire. (She makes people laugh a lot)
* Elle a un humour pince-sans-rire. (She has dry humor)
The best choice will depend on the context and the type of humor you want to describe.