Maintenance

Just like cars, bikes need regular maintenance to keep them on the road. Chains need lubing and degreasing, gears need degreasing, tires need to be inflated, etc. Every bike owner should have a tire pump, at least one spare tube, degreaser, and chain lube. DO NOT USE WD40! There are many types of chain lube and it really depends on your preference. Go to your local bike shop and ask which type they suggest.
If you're biking on a budget, as I am, learning how to fix your own bike is a great skill to have. It requires a bigger initial investment to buy your own tools and parts but it costs a lot less than paying a mechanic to fix it. You also learn how your bike works a lot better and can fine tune it to your preferences. Go to a book store and see if they have any manuals on bike maintenance and repair. I picked up The Bicycling (magazine) Guide to Complete Bicycle Maintenance and Repair for Road and Mountain Bikes and Bike Repair and Maintenance for Dummies. The Bicycling Guide is a lot more informative and in depth but the Bike Repair for Dummies has some great stuff, too. They tell you what tools you'll need for everything and what parts you need for repairs.