Car has sat for awhile

Exactly how long has it been sitting?

For starters, I would drain and dispose of the gas in the tank, and fill it with fresh gas. Check for deteriorated belts, hoses, and tires. Replace the battery since it has almost certainly been killed by sitting discharged for a long time. Then start it and let it warm up, and after checking for leaks take it out for a long drive to get everything up to operating temperature and burn off moisture in the oil.

To check for a head gasket problem, look for discoloration in the coolant or an erratic temperature gauge while driving, or take it to a shop to find out for sure.