Add the user to the wheel group:
gpasswd wheel -a username
I use gpasswd because not all versions of usermod have an easy way to add the user to a group without changing all the users’ groups. However, on any recent Fedora, usermod username -a -G wheel should have the same effect. You could also use the system-config-users GUI, of course.
If you are using Fedora 14 or earlier, use visudo to edit the sudoers file, removing the # from this line:
%wheel ALL=(ALL) ALL
This is the default in the sudoers file on Fedora 15 and newer, so adding the user to wheel is all you need to do.