You will need at least two vents, at least 3" minimum dimension, at least 144 sq inches (with a metal louver, with a wood louver the size will go way up), one within 12 inches of the ceiling and one within 12 inches of the floor, and ... you may need larger openings depending on the size of the water heater (Btu/hr input rating, not gallons).

There are many variables, but the above would be minimums, you may need more.

Ted's advice to have a plumber come out is a plus and a minus ... a plus in that they SHOULD KNOW what you will need ... a minus in that many DO NOT KNOW what you need - this is based on seeing as many water heaters incorrectly installed 'by plumbers' as we do. Either 'they do not know', or 'do not care', what is required - I would hope that 'they do not know' instead of 'they do not care'.