If you're going to stay in Rome, renting a
car won't be of any help. Drivers are dangerous,
parking is rare, traffic is chaotic, and there
are restrictions everywhere. But it could be
useful if you are traveling to destinations
outside Rome. It's cheaper to arrange car rental
before leaving your own country, maybe through
some sort of fly/drive deal.
You'll need to be at least 21 years old, 23
for many companies. You'll find it almost impossible
to rent vehicles without a credit card. No matter
where you rent, make sure you understand what's
included in the price (unlimited kilometers,
tax, insurance, collision damage...) and what
your liabilities are. In some cases you are
liable for penalties in case of theft.