See the following and follow the Battery Replacement links
http://support.apple.com/kb/index?page=servicefaq&geo=United_States&product=ipad
Basically, Apple charges $99 (+$6.95 shipping) for a battery replacement. You also have to ask yourself if the first gen iPad is worth replacing the battery on (it's outdated, slower, will only run up to iOS 5.1.1) or maybe a better option for you would be to put that money towards a newer iPad with more capability. If your iPad does what you want and you feel no need to replace it, by all means, a battery replacement would be for you.