First of all, for most things ITS will not require a user to be logged in under a name. You can just type ^Z and start working. Some programs do require you to have a user name, and maybe even to have a home directory.

To log in and assign a user name, type :LOGIN NAME or NAME$U, where NAME is to be substituted for your user name. Originally, ITS would not as for a password so there was no protection against logging in as someone else. In the latter Arpanet days, ITS was coerced into adding a password facility.

If you don't have a home directory, DDT will complain. This is harmless, but making your own directory is a good idea. To do that, type ^R NAME; ..NEW. (UDIR). The NAME directory will now be yours by virtue of having the same name as the user name. This need not be the case for all users though; see below.