Working on Javascript Form

I am desperately trying to get the Javascript on my billing/shipping form correct, but I am still having a brain-block when it comes to Javascript principles. It's difficult to remember what you need and where, especially when starting from the beginning. Right now my form is just showing the values associated with each field - I am trying to get one field to copy over to another without naming out every variable. I don't think I'm going about this the right way, so I'll probably start over.

Also, I can't remember what (i=0; i < formEl.billing.length; i++) means exactly.

My code:

//will first determine value of checkbox, true or false - check = true or check=false

function validateForm() {
var formEl = document.getElementById("billing_form");
var selectedCheck = copyOver();

return false;
}

function copyOver() {
var formEl = document.getElementById("billing_form");
for (i=0; i < formEl.billing.length; i++) {
if (formEl.checkme.checked) {
formEl.shipping[i].value = formEl.billing[i].value;
}

else { //if false, then leave blank
formEl.shipping[i].value = "";
}
}
}


//code Submit button