Start out by talking to yourself in front of a mirror and imagine that person in the mirror is another person you're trying to speak too.
Keep practicing over and over until you get the hang of speaking to a person without your voice trembling. A lot of people have this problem especially while speaking in public.
You just have to keep practicing and practicing. Once you get used to talking to yourself in the mirror try speaking to a friend or family member doing the same.
Eventually you should have the courage to talk to strangers without being nervous.