OK I'm back
Sorry had to sleep.
It's quite simple. I'll explain how to do it in Sigil, and also how it works under the bonnet. It's a bit of XML in the content.opf page.
In Sigil, look in the "book browser", the skinny panel on the left that lists all the folders and pages in the book.
Choose the xhtml page that you want the book to open on, and right click on it.
Choose "Semantics" from the drop down menu. A dialogue will open with a list of options.
Choose "text" from that list.
This will indicate to the ereader that this is where the text of the book starts, and if all goes well, the ereader will open the book there when it's opened for the first time.
What this does under the bonnet:
Open the "content.opf" page, which is the document that lists everything that's in the ebook. This is the information that the ereader refers to when it wants to see a list of what's in the book, and what each bit is supposed to do. (Sorry, I like anthropomorphisiseing (sp?) tech, helps me understand it)
In there you will see a "guide" section, between two "guide" tags. In the below example, the page that is supposed to open first is called "firstpage.xhtml". See where it says "reference type = "text" ? That's the bit that does the business.
<guide>
<reference type="text" title="Text" href="Text/firstpage.xhtml"/>
</guide>
As you saw from the semantics dialogue box, there are a lot of other options you can use to indicate what the various pages in your book are. For example, you can indicate that a page contains the TOC, or that it's a title page, or a copyright page, etc.
Hope this helps!