It is upsetting to learn you have a sexually transmittable disease and often people are reluctant to tell their partner for a variety of reasons. It is ESSENTIAL that you do as you can pass the disease on.
The first step is to make sure you are well informed about your disease so you can answer any questions that may arise. Be kind, direct and honest and you may get the same in return. Your partner may react with shock, revulsion, pity, anger or denial. Give you both the time to come to terms with what this will mean for you and your relationship.
It may be as simple as seeking medical treatment that will cure the problem or it may mean that you both will have a lifelong, recurring disease.
If you are in a long term relationship, the appearance of the disease may raise the subject of infidelity. This is a topic that often leads people to seek help in sex / relationship therapy. It is painful to come to terms with broken trust but with help you may find that your relationship becomes stronger than it has ever been before.
If you are about to have sex with a new partner and you have an STI: do tell your partner before anything sexual happens.