There are plenty of good answers here already, but I'll add my :twocents: just for good measure.

While distilled water would be the best for germinating your seeds, there are other ways of cleaning up regular tap water to provide for an almost sterile environment for your seeds.

Method #1: Put some regular tap water in a bucket, and add an airstone to the bottom of it. Hook the airstone to a pump, and leave it running in the bottom of the bucket for at least 24 hours. This will oxygenate the water and remove the chlorine that might be present. (If you are using water from a city's municipal water supply, it probably contains chlorine.)

Method #2: Take a smaller amount of tap water (1 cup) and add a few drops of pharmacy-grade hydrogen peroxide, and let it sit for a few hours. The peroxide will remove many of the impurities from the water, and is also rumored to prevent 'damping off'.

Method #3: Just germinate with regular tap water! There aren't excessive amounts of impurities in most localities water. Don't get me wrong - there are some, and in some places, probably noticeably more impurities - but, probably not enough to kill your seeds. Granted, regular tap water might inhibit initial growth...but it shouldn't all-out prevent the seeds from germinating.

If all else fails - just make a trip to Walmart whenever you can. You're guaranteed to find some distilled water there.