Once you have found the property you want to buy, you must ensure that the vendor cannot sell the property to another purchaser. To cover this potential problem you and the vendor must sign a contract that binds the vendor to sell and you to buy. It also allows time for you to finance the purchase and collect the documents needed for the final deed.
Considering the importance of this contract, it is advisable to ask a professional to draft this agreement. His/her professional experience will serve as the best guarantee against any loophole.