Photoshop Tutorials - Stroke Command
You can stroke the selection in Photoshop with the help of the Stroke command that is located in Edit menu. The menu of this command consists of several sections: Stroke, Location and Blending.
Stroke section is very simple. The width option lets you set the width of the area that will be stroked.
Location sets the location of the stroke compared to the selection border: Inside, Center and Outside.
Opacity and mode settings in Blending section are already familiar to you.