Hi ,
First Register The User Control By Adding This Code On aspx Page :
<%@ Register Src="~/WebUserControls/ucPostList.ascx" TagName="ucPostList" TagPrefix="ucPostList" %>
Secondly Create a Object in Cs Page and find the Control :
ucPostList objPostList;
objPostList= (ucPostList)this.Page.FindControl("ucPostList");
After Creating the Object you can Call any functions Or Methods from your User Control.
objPostList.myFunction()
Hope This May Help You.