What must I enter into Terminal, using sudo to make myself an administrator?
If the actual user &lt; user_name > has a standard account, you would have to enter

sudo dscl . -append /Groups/admin GroupMembership &lt;user_name&gt;


to make &lt; user_name > an admin.

Only a restricted number of users are sudoers (i.e. accounts which are allowed to run su or sudo with root privileges successfully) though. The standard sudoers file (/etc/sudoers) in OS X looks like this:

...
# User privilege specification
root    ALL=(ALL) ALL
%admin  ALL=(ALL) ALL

# Uncomment to allow people in group wheel to run all commands
# %wheel    ALL=(ALL) ALL
...


So only root and members of the admin group are allowed to run sudo by default. The above command run by &lt; user_name > will fail consequently, because  &lt; user_name > isn't in the list.

To enable sudoing for &lt; user_name >, you would have to add &lt; user_name > to the list (below # User privilege specification) or uncomment the %wheel line with sudo visudo which has to be run by sudoers again (that's the Catch22 mentioned by Tetsujin).