A dark, rainy night is the worst time to fix your first flat tire. You will certainly need to do other repairs over the course of many months of bike commuting. The encouraging thing is that most repairs you can do yourself . A bicycle is a far simpler machine than an automobile. With the help of more experienced cyclists, magazines or a how-to book, you can easily learn to fix your own bike.
Many bicycle shops offer workshops or classes in bike repair.
I took a class at the Freewheel in San Francisco, a community bike shop that provides tools and repair stands for members to work on their bikes. It's a very satisfying feeling to be a self-sufficient cyclist. Keep your drivetrain clean and your bike will work well and you'll be a happy commuter. A stretched worn chain causes your rear sprockets to wear down to match the chain. Replacing a freewheel or a cogset or even chainrings can get a little pricey.