Instructions
Log into the Xbox Live system with your gamer tag. If multiple players use the same account, press the large button in the center of your controller. Select the "Sign In" option and choose your gamer tag from those listed.
Hold down on the center button until the Xbox menu appears. Move the joystick over until you locate the "My Friends" section.
Scroll down to the bottom of the page and find the gamer tag search function. Enter your friend's gamer tag in the box and press enter or search.
Wait for the results and send a friend request to the gamer tag associated with your friend. If you find multiple results, select the profile associated with each name and find your friend.
Try different names and variations of your friend's gamer tag if you cannot locate him. Even misspelling one part of a gamer tag may result in no results found.