Most book designers use a separate file for the HTML TOC and add the proper semantics to it. However, if the whole book is stored n one file, you'll need to add an id to to first element of the TOC or the element immediately before it (e.g
id="toc") and manually add the value of the id to the guide section in the .opf file. For example:
Code:
<guide>
...
<reference href="Text/book.xhtml#toc" title="Table Of Contents" type="toc" />
</guide>
Since this might be a bit too complicated for you, it's probably easier to simply separate the TOC from the rest of your book by inserting Sigil split markers immediately before and after the TOC by pressing CTRL+SHIFT+ENTER. Then select Split at Markers from the Edit menu and add semantics to the new TOC.
(Make a backup copy first, before you attempt the above steps.)