A good interpreter must have some good qualities. He should, for example, be friendly, cheerful and helpful. Everything he does contributes toward creating an impression in the other people's minds. He must be a good listener and intrested in what he does.
Besies, he must be accurate in what he says and possess fine speaking skills.
People need an interpreter because of language difficulties. That's why they want someone who is not only able to change words into another language, but who also has some good behaviour chracteristics such as sincerity and trustworthiness. They need someone someone who is sympathetic to people.
So, an interepeter's character or personality is as important as his language proficiency .
So what you need-in addition to your diploma- is to have those qualities. Also, you must have high self-esteem and be optimistic about the future. :)
Good luck!