I want to add myself (the only user) to the sudoers file, but I haven't learned how to use vi, which the file says must be used to edit it. I don't suppose there's another way?

Unless visudo(8) was compiled specifically to use a different editor, vi is what it defaults to. (You should probably learn vi.)

If you're willing to forgo syntax checking, you can edit it with whatever you'd like, e.g.:
# nano /etc/sudoers

Just so I'm being clear: I would recommend the former approach.

It is possible to edit the /etc/sudoers file directly, but visudo is recommended because it will check your changes for syntax errors before committing them.

But it's not necessary to do it directly, because visudo will obey either the VISUAL or EDITOR environmental variables if they exist (except in the unlikely event you have a version that wasn't compiled with such support). Just set one of these to the editor you want, and it should use that instead of vi. See the visudo man page for details.

(Many other programs use the EDITOR variable too, so I recommend you set it to your favorite text editor anyway.)

You should use visudo. This will ensure correct file locking and syntax checking. By default, visudo will probably use the vim editor, but you can change that by setting the EDITOR variable.

For example if you want to use nano, run the following command

Cheers,

Evo2.

for me, on debian, visudo uses nano out of the box
(which is very confusing for me: i hit an "i" or an "G" or an "$" and it prints it on the screen. wtf, where is the command mode?. lol). if its the same way on mepis i wouldnt know.

if you wanna learn vim install the full version:
which is a bit more comfortable, less confusing.

with the following you should be able to use the basics within a few days:
http://www.swaroopch.com/notes/Vim_en:Table_of_Contents

at least you only need two commands/shortcuts:
i to change from command mode and be able to insert text at the cursors position.
hit the esc button to get into command mode and x to save the changes and quit vi
(esc and q! to quit without saving the changes).
use the arrows to navigate for now.
(hit esc + then u deletes the last changes you made, which might be necessary now and then)
it might be better hitting the esc a few times, so you will for sure change the mode.