HTML - Use selected attribute.
<div id="tabContainer" dojoType="dijit.layout.TabContainer"
tabStrip="true" style="width: 100%; height: 20em;">
<div id="tab1" dojoType="dijit.layout.ContentPane" title="Tab 1">Tab 1</div>
<div id="tab2" dojoType="dijit.layout.ContentPane" title="Tab 2"
selected="true">Selected tab 2</div>
</div>
JavaScript - Use selectChild method on TabContainer widget.
var cp = new dijit.layout.ContentPane({
title: 'Tab title',
content: 'Selected tab...'
});
var tc = dijit.byId("tabContainer");
tc.addChild(cp);
tc.selectChild(cp);
You can find more examples here: TabContainer Demo
WARNING!!! This demo is from nightly build. Not all features are included in 1.3.2 version.