What must I enter into Terminal, using sudo to make myself an administrator?

If the actual user < user_name > has a standard account, you would have to enter

sudo dscl . -append /Groups/admin GroupMembership <user_name>

to make < 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 < user_name > will fail consequently, because < user_name > isn't in the list.

To enable sudoing for < user_name >, you would have to add < 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).

That's a bit of a Catch22…

You need to already be an administrator to be on the sudoers list [with very few manually-changed exceptions]

Essentially, you cannot promote yourself.
That's the entire point of sudo & being an administrator, to be able to assign non-admins limited abilities.