I am getting my first order for a wedding cake for a full paying customer. I know I need a contract but don't have a clue how to write one. I also don't know when to ask for the deposit or how much. The wedding isn't until Feb. Any advice or sharing would be greatly appreciated. I want to be as professional as possible so that I will hopefully get referrals.

there is a great sample available at earlenescakes.com

Be sure you have a legal business, other wise the contract cannot be enforced.

I have posted my contract here for CCer's to copy.
www.SweetArtWeddingCakes.com/Contract
As for how much of a deposit, that is up to you. Some charge $50 or $100 to reserve the date, others charge a percentage of the cake total, like 25% or 50%.