A good place to start is checking with the BBB (better business bureau), they can save you from a bad choice. Be sure to ask for references and make sure they have phone numbers so you can check them out. Insist on seeing a written estimate with details on the materials to be used, manufactures names, product names etc; That way you can compare apples to apples. You should secure a minimum of three bids, more the better. DO NOT just shop price, "The bitter taste of poor workmanship lasts longer than the happiness of that cheap price". If the deal sounds to good to be true it probably is, the estimates that are good are the ones that are 100 to 500 apart. 

Does the contractor have a brick and mortar store, or just a phone number? Is the contractor certified by a shingle manufacture? When you get your estimate ask for a copy of their workman's comp and liability insurance, this is important and any reliable roofing contractor will gladly supply you this information. 

Some things to ask for in the estimate, Shingle Manufacturer, Underlayment type they use and who is Manufacturer, The name and manufacturer of the Ice and Water shield, Do they use Aluminum drip edge on the eave and rakes, what type of pipe flanges do they use, be sure to ask about ventilation ie; Ridge vent type and manufacturer, Power fan, passive air vents and what ventilation do they recommend. Ask for a copy of the Manufacturers warranty and does it include workmanship? Once you have done your homework ask them to update their estimate to include the products you want, apples to apples. 

Remember the roof is the single most important thing on your home, you should only have to do it once, so do it right. Good Luck and hope this is helpful. Anthony Montagna,