Use the database launch properties dialog:
Note that in this instance, I tell Notes/Domino that I'm using a Navigator. There is a Navigator named "calendar.html" in the database, but it's blank — I'm actually opening a Form named "$$NavigatorTemplate for calendar.html" so that I have access to fields and form events. The Navigator is not embedded on the Form, but it does need to exist in the database for this to work.
You can also set the database to open to a Page, but I've found that forms are far more versatile in general. Pages are great if the information is pretty static, but if the content needs to change, a Form is a better idea, and the easiest way to open a form (other than for editing) is to make it a $$NavigatorTemplate or $$ViewTemplate.