Courage is the main thing you must bring IMO. Genetics can sometimes tell secrets that are difficult to face. Be prepared emotionally. Make sure your partner is prepared as well and that you two have a good understanding what each other do and do not want to know... remember that the geneticist 'thing' is to find answers though your blood, there are 'councelers' involved too but the geneticist is a scientist at heart who likes to figure out riddles... if you don't want the answer or only part of the answer you can always say no...
If you/they know what thy are looking for specifically then it's basically blood work then just being prepared to accept the results, whatever that takes for you... just your partner, or family, friends, clergy, the guy on the corner... us folks here! that can talk you through some of your emotions and help you understand what it all means.
If it's looking for the needle in the haystack type of testing... talk to your family and your partners family, try and get a list together of blood relatives that have any abnormalities, even slight, things that might of been odd that usually isn't talked about. Any miscarrages, stillbirths, young deaths... diseases and sicknesses.
I wish you luck and I hope you find or don't find what you are looking for!