Your profession is gained after serving an apprenticeship in a guild and choosing to join the guild as a full member. At that point, you will assume whichever profession that guild is associated with (see HELP PROFESSIONS). Once you have taken a profession you will be tied to it, and your skills will develop according to the specialities of your guild.
Changing profession can only be done at great personal sacrifice using the CHANGE PROFESSION command. If you choose to reject your profession you will lose all of your specialist guild skills, and the time spent learning will simply be deemed to have augmented your general skills, at three-quarters of their effectiveness. Under normal circumstances you will need to QUIT GUILD before entering the CHANGE PROFESSION command, to declare your departure from the guild before casting aside the professional skills. This is not the case for those bound by the bondsman's oath (see HELP BONDSMAN) since quitting the guild is not an option. For the bondsmen and women, you gain direct access to CHANGE PROFESSION to cast off both your guild and your profession in one declaration.
WARNING: your general skills though will not be able to progress beyond Ultimate, so it is possible to actually lose skills that would have been gained on general skills that are Ultimate or above. You will then be free to join a new guild and start afresh.