>> First of all load the js file like
<script language="javascript" type="text/javascript" src="jsfilelocation/filename.js" />
>> Remeber to return true / false based on the condition in your validation function.
For example, in your js file, let's say there is a method called Validate() that validates the textbox. SO the function should return true / false
function Validate()
{
...
}
>> Now in your submit button write...
<asp:Button ID="btnSave" runat="server" CssClass="ActionButton" Text="Save" ValidationGroup="vgAddress"
OnClick="btnSave_Click" OnClientClick="javascript:return Validate();"