Hello Ravi,

In order to select/deselect a node client side, you could get a desired node like

var tree = $find("WebDataTree1");

var node = tree.getNode(0);

and use node.set_selected(true) and node.set_selected(false).

However, if the requirement is to fire the client and server side events of this selection, I suggest calling the _selectNode with the click browser event.

var tree = $find("WebDataTree1");

var node = tree.getNode(0);

tree._selectNode(node, true, "click"); 

I am attaching a sample for your reference. In my application I am handling Initialize client side event of the WebDataTree. In this event I am selecting the first node as the requirement is. In this scenario SelectionChanging , SelectionChanged client side events as well as Selection changed server side events are fired. 

In order to conditionally select a node Server side, you could reference to the following forum thread:

http://ko.infragistics.com/community/forums/t/48590.aspx

Please let me know how my suggestions work for you!