Depends on what you mean by "get my feet wet". If you really mean, just "see how it works" or "make a purely toy app", then use XMLHttpRequest to learn the underlying gunk. If you mean "write a simple but useful/real app", use Prototype or jQuery (I'd pick jQuery). If you mean "write a complex/real app that I will want to evolve and maintain going forward", use jQuery or GWT. But, GWT is really a whole different ball-o-wax and has some major advantages, but it's also got some issues. Script.aculo.us is also worth a look.