First, decide what kind of ceremony you want and whether it needs to be legally binding on the day. A registered solemniser can legally marry you; some couples handle the legal element separately and have a celebrant lead a personalised ceremony. Either way, meet or speak to your celebrant beforehand to make sure their style suits you.
Ask how much the ceremony can be personalised, whether they help write it, how far they travel, and whether they are available for both your date and your rehearsal.