The content stored in an Alfresco repository is described by a so-called data dictionary. You have to extend that data dictionary with a description of your additional properties (name, type, multiplicity, etc.). There is a guide in the Alfresco wiki.
You must tell the web client to display your additional properties and/or to make them editable. The Alfresco wiki provides another guide describing how to achieve this.
To get an overview over all pages in the Alfresco wiki related to content modelling check this.