How can I tell someone I have hepatitis c?

Honesty. Hepatitis c is for the most part only passed by blood. Most patients acquired it via experimenting with drugs, blood transfusion or tattoos. You do not need to tell most of your friends that you have hepatitis c, but should probably tell sexual partners.
With your doctor? Sometimes i will join a patient who needs to tell a partner that they have hiv, hepatitis c or hepatitis b infection. It can help to have someone who can answer questions. Tell them how you found out, that you are in medical care, and that hepatitis c is curable. Describe how you will keep this person safe so they do not get it (see some of my other posts). Use condoms for sex with a new partner.