How to Build Your Credibility

1. Advertise your competence - At the beginning of your presentation, tell your audience about your expertise on the speech topic. If you have done a lot of research about the topic, tell them so. If you have a certain experience that gives you special knowledge or insight, go ahead and say so. But keep in mind; you don't want to sound boastful to your audience. Do not over-advertise yourself. 2Keep it short and simple. Say it as a matter of fact, not brag. 
2.  Connect to the audience - Try to identify with your audience early in your speech. Even if you are going to talk about something very controversial or something your listeners may disagree with, you still have to make them feel you share the same common ground and values.Years ago, Senator John Kerry gave a speech about keeping women's rights to abortion in one of the "red" states, in a room full of conservative voters who were probably strongly against such an idea. At the start of his speech, he made a smart move by saying that he himself is also a true Christian who believes abortion is not the right thing to do. Then he explained that even though that is what he believes, there are also a lot of people in America who do not share his religious principles. And since America is a democratic country, we have to respect those people's values as well. By establishing common ground with the audience early, he was able to get off on the right foot. I don't know how many people in the audience he had successfully convinced, but at least he pulled off that extremely controversial speech with such poise and more importantly, without getting booed 
3. Speak eloquently and express your ideas with conviction - Practice your persuasive speech ahead of time so that you can perform it well. Moderately fast speakers tend to be considered more intelligent and confident than slow speakers. If you sound hesitant or say "uh" and "um" too much, you will appear less competent. 
4. Use evidence - For amateur public speakers with no initial credibility, it is very helpful to use examples, statistics, facts or testimonies to support their ideas. No matter what type of evidence you use in your speech, try to remember these two things. First, use specific evidence. For example, if you use statistics, indicate the exact number. Stating "Ten million Americans suffer from obesity" will be more effective than just saying "Many Americans suffer from obesity." It will make your listeners aware that you have a firm grip of factual information. Second, always cite evidence from well-known, reliable and non-biased sources. 
5. Reason clearly and persuasively - Even if you use a bunch of strong evidence, you still won't be able to persuade your audience unless they grasp your reasoning. Don't assume that supportive evidence is enough. Throwing a lengthy list of statistics and examples at your listeners without drawing a logical conclusion to your main idea won't do you any good.
6. Appeal to emotions - Some people say that serious public speakers should avoid emotional appeal entirely and only stick to reason. I disagree with that. Humans are not like automatons or Mr. Spock in Star Trek. We think and feel at the same time. By adding intensity of feeling to your logical speech, you can be a much more compelling speaker. A rational persuasive speech that can change some people's attitudes may not arouse those same people enough to take action. In order to convince your listeners not only to agree with your ideas but also adopt them in real life, you must evoke their passion.