Bank Website - Calculating Loan Repayment With Interest

I am having a few problems with my JS, and now that the module has finished, I find myself struggling still. I am going to post my JS and the HTML page its related to. The site is a mock up for an assignment - basically, its a pretend bank website where a person applies for a loan, and all my JS does is work out if the applicant does the following correctly:

Enters repayment years 1 to 25
Enters a loan amount of 1000 to 250000
Checks a box for loan protection
Calculate button
Reset Button
Then tells the user what it is going to cost them from interest to total amount to be paid.

Javascript:
function init(){
var baserate=4.25;
var amount=parseInt(document.getElementById("loan").value);// end bracket missing - done
var repaymentyears=parseInt(document.getElementById("repaymentyrs").value);// end bracket missing - done
var rate;
var interest;
var loanprotect=5;
var total;
var monthlyrepayment=repaymentyears*12; .....