There are many resources on the Internet for learning how to tie a necktie. On About.com, the video "How to Tie a Tie: Windsor Knot & Half Windsor" shows how to tie two varieties of necktie knots.
The Windsor Knot is a wide and thick triangular knot and is appropriate for business occasions like meetings and job interviews. The Half Windsor Knot is a less imposing version of the Windsor Knot and works well for wider neckties made of a lighter fabric. Other popular necktie knot varieties include the four-in-hand knot and the Pratt Knot. The Pratt Knot is also known as the Shelby Knot.