There is no single start menu as such. The panel that appears by default at the
top of the screen has a number of drop down menus that offer between them
everything the Windows start menu does and more. The Ubuntu symbol at top left
is the Applications menu. The symbol at top right (mine looks like a power
switch rather than a smiley) has logout, switch user and shutdown options.

To get to a command line go to the Applications menu, then accessories, then
terminal. (There's a hotkey to get there too but, unusually for me, I can never
remember it). You can drag Terminal from its menu and park it on the panel (next
to the Firefox symbol, for instance) if you're going to use it frequently.

To alter the contents of the menus, choose System > Preferences > Main Menu.

But reading your message again, I think you're asking how to use the CLI to
reconfigure the menus? Can't say I've ever seen that done. Being *nix, there
must be some text files somewhere that you could edit, and no doubt someone will
know where. For instance

www.uluga.ubuntuforums.org/showthread.php

seems to suggest that /usr/share/applications contains ".desktop" files that
make up the menus.