Here are a few known causes.
Follow the instructions in the installation wizard to set up database and create your start node. Leave a file here to start ASP.NET when requesting the root folder (/).
If you know you have installed you might need to decorate the content class for the start page node with the [Template] attribute.
[Template("~/Path/To/Template.aspx")]
public void MyStartPage : ContentItem
{
}
The system is configured to not rewrite when the file exists. Since this page (~/Default.aspx) exists the start page template will not be rendered. To fix this problem either (1.) replace this file with the start page template, or (2.) configure <web ignoreExistingFiles="true" />. To secure with ASP.NET authorization use method (1.)
This could happen when the database contains nodes that have no corresponding class. When using medium trust this could be due to an assmebly not beeing defined. Re-start the site and check for any exceptions occurring during the first request.