To configure your system, you need to edit text files. Most of them will be in the /etc directory; and you'll need to su to root to be able to change them. You can use the easy ee, but in the long run the text editor vi is worth learning. There's an excellent tutorial on vi in /usr/src/contrib/nvi/docs/tutorial if you have that installed; otherwise you can get it by ftp to ftp.cdrom.com in the directory FreeBSD/FreeBSD-current/src/contrib/nvi/docs/tutorial.
Before you edit a file, you should probably back it up. Suppose you want to edit /etc/rc.conf. You could just use cd /etc to get to the /etc directory and do:
# cp rc.conf rc.conf.orig
# mv rc.conf rc.conf.orig # cp rc.conf.orig rc.conf
# mv rc.conf.orig rc.conf
To edit a file, type
# vi filename
delete letter the cursor is on
delete the entire line (even if it wraps on the screen)
insert text at the cursor
insert text after the cursor
to write your changes to disk and continue editing
to write and quit
to quit without saving changes
to move the cursor to text; /Enter (the enter key) to find the next instance of text.
to go to the end of the file
to go to line n in the file, where n is a number
to redraw the screen
go back and forward a screen, as they do with more and view.
Practice with vi in your home directory by creating a new file with vi filename and adding and deleting text, saving the file, and calling it up again. vi delivers some surprises because it's really quite complex, and sometimes you'll inadvertently issue a command that will do something you don't expect. (Some people actually like vi—it's more powerful than DOS EDIT—find out about the :r command.) Use Esc one or more times to be sure you're in command mode and proceed from there when it gives you trouble, save often with :w, and use :q! to get out and start over (from your last :w) when you need to.
Now you can cd to /etc, su to root, use vi to edit the file /etc/group, and add a user to wheel so the user has root privileges. Just add a comma and the user's login name to the end of the first line in the file, press Esc, and use :wq to write the file to disk and quit. Instantly effective. (You didn't put a space after the comma, did you?)