A good point to start would be to use jQuery Form Validatior. You can check the documentation and their example by going to:
http://formvalidator.net/#reg-form it has various examples, and they also have a CDN so you can link the library. Do not forget that Jquery when run locally may not display the expected results, so please keep this in mind (due to local machines security features, so you can allways use JsFiddle for testing)