What a horrible way to do things!
If your users have met tab pages before (and they probably have, windows uses them a lot) then they will expect to click on a tab to select the page, not a button outside the tab. Equally, they will not expect other tabs to be disabled. So why are you using a tab control in the first place, if all you are going to do is make the familiar work in a totally different and confusing manner? That is bad for you app, because it means that users have to learn a new behaviour specifically for you, and that is in contrast to the rest of their use of the system.
Instead, if you must do this, then use a set of panels, and make the appropriate panel visible when you press your button, the the others invisible.
If you instist on this, you can set the active tab page very easily - just set the TabControl.SelectedTab property to the appropriate TabPage:
tabControl1.SelectedTab = tabPage2;
But you can't easily disable tab pages - they will still be clickable via the tab itself. So you will need to put a panel on each tab page, and enable or disable that to disable it's contents.