in ur textbox use ... onFocus="javascript:this.select()
eg.
<asp:textbox id="txt1" runat="server" text="Abc" onfocus="javascript:this.select();" >
With this u can select the text of ur textbox
An
onClick="Copy(this.id)"
function Copy(IDS) {
document.getElementById("ur id ").value=document.getElementById(IDS).innerHTML;
}