Clomid for the first time

Hello everyone I am about to take clomid for the first time and I'm happy and a bit nervous. I have pcos and my boyfriend and I have been together for almost 9 years we have been ttc for 7 years now I got pregnant once and had a miscarriage and have not been able to get pregnant again so I have been prescribed clomid so here's my question I have been given instructions on how to take it but sometimes I will spot and then not get  my period for like a day or two later when would day three actually begin. Would it be when it actually comes down or do I count the days I spot plz help