You deserve a great deal of credit for asking this question.
A woman remains in niddah, and is halachically prohibited from having relations with her husband, until she has immersed in a mikveh – even if a long time has passed since her last menstrual period.
Therefore, we recommend that you not wait to initiate the immersion process. For details about practice, please see our article "Niddah In a Nutshell". In your case, you should wait five days (starting today – not with your next period) and then perform a hefsek taharah. For specific questions that arise as someone new to the process, please feel free to get back to us.
If at all possible, we recommend that you approach a kallah teacher, the rabbi's wife, or another knowledgeable woman in your community for some practical personal guidance in this mitzva.