A login name is a sequence of alphanumeric characters and underscores.
You cannot alter your LoginName: you have to get the administrator to do this for you.
How to Find Yours
Look in TWikiUsers - if your name is followed by a dash and a word, usually lowercase, this is what you log in with.
e.g.
TWikiGuest - guest - 05 Sep 2004
Would mean that TWikiGuest would log in using the login name 'guest'
If, on the other hand, if your entry is like this:
