This is covered in the Additional Shortcode attributes section of the plugin’s WordPress.org page and the readme.txt

If you want a specific tab other than the first tab to be open by default when the page first loads, you can add the parameter & value open=”yes” to the shortcode for that tab:

[tabby title="My Tab" open="yes"]

If you use the ‘open’ shortcode parameter in one of your tab shortcodes, ensure that you only add it to single tab as having more than one tab open within a tab group is not supported.