It can be helpful to share your story with them, rather than just your diagnosis.
It may help if they hear how you became infected, if you know this, or what caused you to get tested.
Since hepatitis C is transmitted primarily through shared needles or through blood exposure, it is certainly possible for a person to have a relationship without telling the other person or exposing them to the virus, simply by not engaging in sharing needles or in allowing the person to become exposed to your blood.
However, sharing a toothbrush or razor can put the person at risk, as can any accidental injuries such as a bloody nose, cut from shaving, or other unexpected incidences.
Although they may ask these questions, you can always tell them that you prefer not to answer, however doing so may make it difficult for them to truly understand your experience and what you want them to know.