Negotiating the contract
You need to decide on the minimum (or maximum) price you will accept for the house. You may like to get a valuation report prepared on the house as a starting point for a sale price of your house. A land agent will also be able to give you a good indication of the value.
Depending on how close the buyer's offer is to what the seller had in mind the offer can be rejected or a counteroffer made.
The process continues until both parties agree on the details of the offer or either party walk away. Every time some aspect of the offer changes this releases the other party from the offer they made. You do not have a contract until each party has agreed to each aspect of the agreement.