As Daryl says
Goto this this address
www.YOURSITENAME.com/admin/users/moderators but replace "YOUR SITE NAME" with the your site details.
Or click to enter the admin pannell.
Click on users
click on moderators.
At the bottom of the page you will see a box with the word Username. Enter the username of the account you would like to grant admin powers too and click the make moderator buton.
When the page reloads at the top you will have a list of powers you can grant to that individual including the ability to make them an admin.
Tick the boxes you think appropriate and then click save button at the bottom of the list.
Providing you ticked the box labelled Admin that user user will now have access to the admin pannell.
He will not be able to remove admin abilities from others though at least I don't think he can. That powere stays with the original admin account. Could be wrong though.