The credibility statement establishes your qualifications as a speaker. You should come up with reasons why you are someone to listen to on this topic. Why do you have special knowledge or understanding of this topic? What can the audience learn from you that they couldn't learn from someone else? Credibility statements can refer to your extensive research on a topic, your life-long interest in an issue, your personal experience with a thing, or your desire to better the lives of your listeners by sifting through the topic and providing the crucial information.
Remember that Aristotle said that credibility, or ethos, consists of good sense, goodwill, and good moral character. Create the feeling that you possess these qualities by creatively stating that you are well-educated about the topic (good sense), that you want to help each member of the audience (goodwill), and that you are a decent person who can be trusted (good moral character). Once you establish your credibility, the audience is more likely to listen to you as something of an expert and to consider what you say to be the truth. It is often effective to include further references to your credibility throughout the speech by subtly referring to the traits mentioned above.
Show your listeners that you are qualified to speak by making a specific reference to a helpful resource. This is one way to demonstrate competence.
In doing research for this topic, I came across an account written by one of these heroes that has deepened my understanding of the institution of slavery. Frederick Douglass', My Bondage and My Freedom, is the account of a man whose master's kindness made his slavery only more unbearable.
Your listeners want to believe that you have their best interests in mind. In the case of an informative speech, it is enough to assure them that this will be an interesting speech and that you, yourself, are enthusiastic about the topic.
I hope you'll enjoy hearing about the heroism of the Underground Railroad as much as I have enjoyed preparing for this speech.