The tabcontrol would have a selectedIndex and all you need is to set it to the index of the tabpanel.tabControl.SelectedIndex = 2;
You could also use the SelectedTab property of the tabControl if you have the tabpage object in hand.
tabControl.SelectedTab = tabControl.tabPages[0];
Btw,is it tabpanel or tabpage?
Note: its been a long time since i worked with the tab control. The property names could have slightly varied :-)