Hi Andrew,
Are you referring to a tab on one of your data collection forms, or the home screen (Launcher)?
If you are referring to the Entry Form, you should be able to set this by designating one of the fields on the first tab to be the 'Start on Field'. You can do this in the 'Advanced' settings for the session you are configuring. This will cause the session to set focus to the field you selected on the first tab and also set that tab active.
Let us know if this helps.