Read this thread to start! :) It fails to mention care for tanks smaller than 2 gallons, which DO need 100% changes, as cycling is not stable in tanks smaller than 5 gallons.
How big is your aquarium?
Here are a few example sizes and good water change schedules:
**1 gallon - 100% change every other day.
**2-2.5 gallon - 100% change once a week, 50% change mid-week.
**5 gallon with filter - 50% change three times a week until cycled (need a test kit to determine this!). 50% change once a week after that (gravel vacuum once a week always).
Make sure any water added to a fish tank is conditioned. Prime is a good, cost-effective water conditioner (2 drops per gallon!).