you can see this step by step example to implement live validation in RoR
And always remember that any client-side validation is cosmetical only. As the saying goes - never trust user input. Use live validation all you want, but make sure all data is also checked again by implementing the proper validation in your model, e.g. by using the nice plugin mentioned by Kevin above.
Did I already mention never trusting user data?
It's that important, really.
You can find the docs here: http://livevalidation.rubyforge.org/
ya checked that.. but which .js file to download.. can u just briefly explian... because i am very new to ror and interested in live validation....