You need to add an anchor to the HTML at the point that you want it to open at:
Code:
<A name="start_reading />
and then add an entry to the "Guide" section of the OPF file:
Code:
<reference type="other.ms-firstpage" title="Start Reading" href="book.html%23start_reading"></reference>
If you're using Mobipocket creator, open the OPF file, select the "Guide" page, and enter the following:
Title: Start Reading
Type: other.ms-firstpage
Filename: book.html#start_reading
and then click the "Update" button at the bottom of the page.