How to do something before on submit?

If you have a form as such:

<form id="myform">
...
</form>

You can use the following jQuery code to do something before the form is submitted:

$('#myform').submit(function() {
    // DO STUFF...
    return true; // return false to cancel form action
});

Update; for newer JQuery versions (to avoid deprecation warnings), try:

$('#myform').on('submit', function() {

    // ...

    return true;
});