One of the most essential items to carry. Most can be easily changed to work on either presta or schrader valves by unscrewing the end and flipping the plastic fitting inside. Try it first at home to be sure you know how to use it. Don't count on CO2 cartridges they're great for races where the main advantage is speed. But most riders will only carry a cartridge or two.
Flats are a fact of life.
You are going to get flats. You may as well accept it now and learn to fix them and deal with them. There will be other breakdowns, too. But flats are the most prevalent. There is no bike equivalent of the Auto Club, so self sufficiency is the key.