A contract is simply a document which sets down in writing what you have agreed verbally between you. You should start by writing down exactly what you think are the terms that you have agreed - both from your side and from his. You might want to get down how much you are paying, how much deposit, how much per week/month; I would also include an appendix of all the addresses. you might want think about what happens if things go wrong - such as non payment or if he starts to approach the custies afterwards; you may also want to think about clauses which prevent him from selling the round to others.
if you want I could help if you want to send me a list of the terms you want to include
Cheers, Tom