Make sure the company is equipped to do the job you need done. For instance, if you need a large tree removed, do they have a crane? Will they grind the stump? Will they remove the tree and debris? Make sure the company is properly insured. Ask for recent references. Check their rating with the better business bureau.
Confirm it is understood if removal and clean up are included in the quoted price. Once all aspects of the job are agreed on, make sure to have the details written down in a contract that both parties agree to.