How to debug an event of new line (enter) in HTML

I'm a newbie to web programming. I've downloaded a free template of html responsive website & there is a funny behavior I'm trying to debug and find the source/cause

the site has a textbox and a button in a form -- every time I press enter on inside the textbox the website "reloads"/scrolls to the top linked a function to the onkeyup event and tried to do event.preventDefault(); if the event.keyCode == 13 but it did nothing

How can I debug and understand the flow within the files and different functions of the site once an enter is clicked.

Improtant Note: I don't have an IDE and am working with a text editor and the Opera browser.