hi !!!
you need to select an insurance company first like AOK for example. i picked the one that had an office that was closr to me. tell your employer about your decision. i heard some employers do the registiration process for you but if you have never been insured before, you can just go to the health insurance company with your vertrag and say "i want to be insured by you and i am working at this x job", they will help you. just make sure to let your employer about your health insurance company of choice and let them know after you have gone to the insurance company and register