how can display please wait message on the button

This can easily be achieved by using the ClientScript.GetPostBackEventReference method.

this.Btn_Upload.Attributes.Add("onclick", "this.value='Please wait..'; this.disabled=true;" +
 ClientScript.GetPostBackEventReference(this.Btn_Upload,""));

The GetPostBackEventReference injects the __doPostBack method and
 also the hidden fields that hold the event target and event arguments.
This is a useful technique when
uploading large files and during the process disabling the button control.