Just like a car, your bike needs to be maintained on a regular basis. If you go too long without cleaning and lubing your chain, you'll start to notice a squeaking sound that will get worse over time. First, wipe your chain down with a shop towel (or a piece of an old T-shirt), then use a bottle of bike-specific lube (not WD-40) and apply a drop of lube to each roller and pin. Spin the crank slowly as you work your way around, then wipe away the excess lube with a clean towel. Don't overdo it—you should only lube your chain once a week or every 200 miles.