First, let's look at the default behavior when a document library is selected in SharePoint. In this example, I select my Shared Documents link on the left hand navigation and directed to the Shared Documents document library (the link is: //server/shareddocuments/allitems.aspx):
Notice how I must then select the Documents or Library from the ribbon bar at the top. Many of our clients want to go directly to the Documents or Library option. In this example, a link is created to direct the user directly to the Documents tab:
Now when this link is selected, Shared Documents opens with the Document tab active:
To default to the Library tab, just set the URL to this:
Now when this link is selected, Shared Documents opens with the Library tab active:
It is rather simple, just adding the ?InitialTabId=Ribbon.Document to the end of the link to the library.