You don't need to disable the submit button, you just need to show an error message and stop the form from submitting.
Try using http://jqueryvalidation.org/ to manage your form validation. You would just mark required fields as such and the validation framework will make sure that the form doesn't submit until all rules are passed.
<input type="text" name="firstName" id="firstName" required />
That's all you need to do with jQuery Validate.