403 means Forbidden. Maybe the website requires you to log in before you are allowed to view the webpage. It's also possible that the website detects that you are not a regular browser (maybe by looking at the User-Agent header) and block you, because it doesn't want automated bots to be able to read the page.
If you need to log in, you'll have to do more in your program than just try to read from the page. It's most likely easier to program that using a library such as
Apache HttpClient - Java's standard classes for reading webpages are quite underpowered and don't (easily) allow you to do things like logging in on a website.