Travis Jordan wrote:
"You need a new roof".

OK, seriously, be sure to ask about the preparation of the roof decking
prior to shingling. Are you going to renail (a common early technique
was to staple roof sheating in place, this is one reason you see roof
panels blown off in every tornado and hurricane damage photo)? How
about ice-and-water shield if you are in a cold climate, or secondary
waterproofing, a peel-n-stick membrane over every sheathing joint if you
are in a hurricane prone area?

What kind of underlayment will the roofer use? Is it 15, 30, or 43 lb
felt, or something better such as an inorganic fiberglass base sheet?

How will the valleys be done, open or closed? Will the contractor use
metal flashing for the valleys, and if so, what kind of metal?
Galvanized steel is common, galvalume is better, copper is best. Will
he cover the valley metal with underlayment, or leave it exposed?

If you have a chimney, will it be reflashed? What technique (ie.
counterflashing?).

How about ventilation? If adding roof vents, are they approved for the
conditions in your area (turbine vents are a bad idea in hurricane
country, unless you have a storm cap for each one of them and install
them religiously before any storm is headed your way).

Google on roofing basics before you meet with the contractor(s) and
you'll be a more informed consumer.