Why should I speak and about what?

Speaking gets you noticed and it builds your reputation. It's a great way to give back to your community and peers. You will sharpen your abilities and get exposure to new friends.

You can speak about anything that you think will interest a particular group. Just make a list of topics and let the meeting organizers decide on which to pick. Whether you've got decades of experience or are a new graduate, you know many things that others would like to know more about. Think of all the things that interest you professionally or personally. Things you did at work that others would benefit from learning. Things you learned in school. Your hobbies. So on.

It doesn't have to be a hour long seminar. You could put topics up that are just 5 minutes long. There are organizations who just want several people for few minutes as say the 'opening band' if you will. There are other settings where there's a panel of speaker type of arrangement.