In the admin control panel, in the left menu go to Permissions under User Admin. Type in the username of the user that you wish to make an administrator and click Look up user. At the top of the page will be a drop down menu that has User and Administrator in it. Select Administrator and click the Submit button.
If you want to give this person a user title to let other users know that he or she is an administrator, you can do so by clicking Management under User Admin in the left menu. Type in the username of the user and then click Look up user. Scroll down to Rank Title, select the administrator rank title that you created and press Submit. If you do not already have a rank title for administrators, click on Ranks under User Admin and click Add new rank. Type in the rank title, set as a special rank, skip minimum posts and denote a rank image on your server if you would like (enter the path to the image on your server from your phpBB). Now, you can go back and give the user in question the rank title that you want.