Steps to go on Clomid?

Kimberly
My gyno is finally going to put me on Clomid. She said that I will need to come in on the first day of my period but that could be a problem with their special holiday hours so I might not get in until January. She left the info in a voicemail so I didn't get all the info. Why do I have to wait for my 1st day of the period before I can go in and get it prescribed? Why can't I get it now and take when I'm supposed to? Do they have to do an ultrasound or something first?