As a general rule, allow at least an acre of land per horse. Check the field for health hazards (rubbish, pesticides etc.) and ensure that the field is properly fenced. The field should be fenced so the horse cannot escape or injure itself. Do not use barbed wire.
Your horse needs to be able to shelter from the heat, cold and rain. A three-sided shelter / run-in shed will work very well. The flooring of the shelter should be dry and the walls should be strong and hazard-free.
If your horse is kept in a stable make sure you provide adequate bedding (e.g. straw, hemp) and keep the stable clean by following a daily mucking-out routine.