We had a thread a while back where everyone talked about their favorite border method, but it may be difficult to find via search. A quick Google search, however, brings up this article as the first hit.

There are millions of Photoshop tutorials, if you're ever wondering how to do something you can likely find instructions on Google.

P.S. - In future, you'll get better results if you have a descriptive thread title. For example: "Can someone help me add a border in Photoshop?"