When an author is trying to come up with a topic for his next story or novel, the old pros in the writing came will always give him the same advice.
"Write about what you know." That is because if you speak from your own area of expertise, you will speak with authority and passion. And authority and passion not only make for a great story or novel, they make for a really good public speaking event as well.
When you are putting together what you will use for your talk to that group you want to amaze, you want both of those elements, authority and passion.
But on top of that, you have to give them something to make it perfect. You have to tell them something they don't know.
To achieve a balance of what is familiar with what is new and fascinating will be the stuff of your research and preparation for public speaking.
Sometimes telling them something they don't know might be just bringing a new joke that they have not heard. Or you might bring a fascinating story or anecdote that will lead directly into your talk.
That can grab their attention and let them know that this is going to be an interesting take on the subject. Finding jokes that nobody has ever heard before can be a challenge. But that is ok because canned "jokes" are not best for your speech anyway.
It is much better to find a funny or very amusing situation that relates to the topic from your past. By telling the story of that situation with plenty of self referencing humor and commentary, you can have your audience very amused as you move into the body of your speech but at the same time very interested in you and so in your topic.
Sometimes finding material that is new to your audience is obvious and easy to identify. It might be that you were invited to give the speech because you have some expertise in a subject that your audience wants to know about.
If you are giving a speech about how to make your own PC from scratch and you know a lot about that, you are in good shape right off the bat. Your listeners are sure to learn plenty from your presentation and have lots of questions for you after your talk. You told them something they didn't know.
However, if your topic is a little more in the area of common knowledge, you might have to do some research to find things to share that will get those eyebrows to raise.
One rich repository of little known facts lie in what we call trivia and urban myth. You might be giving a talk about the internet. Now most of us know quite a bit about the internet.
But with a little research, you can uncover a lot of trivia about how the internet came to be, how the internet actually works at a structural level or whether or not Al Gore really did invent it (he didn't).
But the internet is also a great topic to go out and pull in literally dozens of urban myths that will make for a very enjoyable presentation. From how viruses work to whether or not that African prince really will send you 5 million dollars or not can give you lots of things to share that your listeners probably did not know (incidentally - he won't).
So approach your research both to fill your speech with good solid content but also to include information that may be amusing or anecdotal to give your listeners something to talk about over coffee later on. If you make your speech that memorable, they will think of you as a great speaker and probably ask you back again.