TagVal('yourTag', '*-12h') should do it. You can find out more about the Performance Equation (PE) expression syntax - this is what's used by ProcessBook data sets - in the \PIPC\Help\PEReference.chm file.
With that said, I would like say
1) Welcome to the PI world! (you said it's your first exposure to PI software)
2) This kind of question is better handled by our regular technical support team. They are much better equipped (and staffed) for end-user and PI admin type questions. The OSIsoft vCampus program is intended for programming and integration type topics - i.e. developing custom applications, integrating PI with other business systems or databases, etc.
Hope this helps!