How about Drinking and Driving? Or…

Smoking?

Drugs?

Peer Pressure?

Poverty in the US?

Think of things that are affecting young people and yes, even older people.

Hope that helps kind of give you some ideas.

I took a public speaking class last year and I found that the speeches that people where more conected to where much more interesting, For example my neice was just diagnosed with crohns disease so that was what i choose for my speech (of corse i narrowed it down its a pretty broad topic to cover). Sometimes something that you are connected to is something hard to think of so if you cant do that try doing your speech on something out of the ordinary, there was another girl in the class who did her speech on savontism (im pretty sure im not spelling that right, sorry.) It might help to first decide on a very broad topic then narrow it down to what part of that topic you actually want to write/talk about. hope this helped at least a little bit.