Ladies I have some questions !

So I’m currently two days late on af and that’s very abnormal for me as usually I get signs of it coming a week prior and being on time to the day. My obgyn prescribed me clomid but from what I understand I can’t take it until af starts ? When is usually a good time to take a test, like how many days late and or does first morning matter or not ? I’ve tried reading up online but there’s so many different sides to hear pov from. Any insight would be highly appreciated!