Call several drywall contractors and ask them to place bids or to give you estimates for the repair to your drywall.
Find out the going rate of drywall work, this number will vary depending on the area in which you live and the skill of the contractor.
Find out the reputations of the contractors. This can be done through friends who have had work done, or by asking for references from the contractors.
Choose a contractor based on skill and cost.