If you have any Meetups (or similar groups) in your area for any topic you are interested in / knowledgeable about, then contact them. Most such groups are always looking for speakers and will happily accept anybody who volunteers. And the expectation aren't usually super high from the audience. So if you have a few too many "uuuhhhs" and "aaaahhhhs", or your slides are a bit janky, or it takes too long to get your laptop to connect to the projector, they'll usually be pretty forgiving. Nobody is going to throw tomatoes at you or anything. About the worst thing that would usually happen in that setting is that some impatient person gets up and walks out.

Local meetups like this are a great way to cut your teeth at presenting and public speaking. And yes, I speak from experience here. I did the firefighting instructor stuff mentioned above in my early to mid 20's but after I left the fire department behind and moved to a new area, I got involved in the Meetup / User Group scene and started doing a lot of these kinds of talks. Both experiences combined definitely helped me overall in terms of becoming a (somewhat) confident public speaker.

Toastmasters is also an option I hear good things about, but I have no first hand experience doing that.