Word is a style based application, though many users strive to avoid using the styles, favouring manual formatting and get themselves into all kinds of avoidable trouible as a consequence. Each paragraph should have a style applied to it that defines the format of that paragraph.
The table of contents function, by default uses the names of the built-in heading styles which are uised to format your headings and sub headings (suitably re-configured to provide the appearance you require).
However you can also link other styles to the table of contents, or individually mark items to be included.
See
http://www.gmayor.com/table_of_contents_addin.html for more information and a function to make life easier for those working with TOCs.