Once you have received an offer, you can start negotiating. Take your time and do not be rushed by the buyers. Consider all aspects of the sale, deciding whether you want to negotiate on the price, timescale for exchanging contracts, and what else you may want to include in the sale price such as fixtures and fittings.
Once you have accepted an offer you should start preparing to exchange contracts. You are under no obligation to sell until you exchange contracts. However once contracts are exchanged and signed you are then legally obliged to sell the property. The moving date can be decided between you and the buyer. Leave the property in a good condition, cleaned and ready to welcome the new occupants.