Form submission using Javascript?

I am trying to write this code so that when someone try’s to submit this form, it will not let them unless their e-mail, first and last name fields are filled in. Right now I have it so an alert box pops up if the feilds are not filled in, but then it continues to process the form? If someone could please take a look at my code on my site, maybe you will be able to see the mistake I am making. The link is below.

http://boebarlage.limewebs.com/rec_application.html

Any advice will be greatly appreciated!

Boe Barlage