Re: Giving admin rights to existing user
Yes, It is generally accepted that logging in graphically as root is a bad idea (not wishing to open that debate again...).
Use su or sudo to run specific programs or a shell as root, or log in as root on a virtual terminal (Ctrl+Alt+Fx).
Being a member of the "wheel" group makes using sudo possible, as PabloTwo explained. This is all that making your user an "Administrator" when installing actually does for you. You can go to your status menu (user name at top-right of the screen)->System Settings->User Accounts (at the bottom of the Window)->Unlock->Account Type to change your account to an administrator (i.e. add it to the wheel group).