To gain confidence speaking I suggest starting small and with a non work related subject you really enjoy. Do you enjoy kayaking? Organize a quick presentation with your local club about different ways to secure it to your car. Into motorcycles? Do a local motorcycle club presentation about how to change a tire.

As far as topics, try picking something you just did you are proud of, or something you'd like to get better at. There is no better way to learn a subject than to try teaching it to others, especially if those others are experts in the subject.