Just try to give an ID to your TabPanel - in my example named "MyTabPanel", given ID "MyTabPanel"...
got one window 4 panels in it, window without ID, tabpanel with ID "MyTabPanel"
write to console
Ext.getCmp('MyTabPanel').setActiveTab(0); //for first tab
Ext.getCmp('MyTabPanel').setActiveTab(2); //for third tab etc.
if you want to do it by default (i.e. whenever you open your project preview) just put into tabpanel options (in Designer - lower right side by default) "activeTab" to 0 (or any other number) ... more info is here