Sign in to Bing webmaster tool,
You can use your existing MSN/Hotmail account to login, else create a new one.Once you are logged in, click on add new site.In the next page, submit all the details, and click on submit.
Depend on your preference, you can select any of the two method. Uploading XML file is much easier option , download the xml verification file and upload it to the root of your site.