Open Office Impress is a presentation program just like Microsoft Powerpoint.
Its main advantage is it's free.
Impress software has another advantage. You don't have to use a Microsoft product anymore. Of course this is only an advantage if you care about that sort of thing!
Otherwise for most of us, it's about the same as Powerpoint. ( I'm sure the geeks among you are screaming at this point because it can export to .swf and PPT can't.) No doubt there are other subtle bells and whistles which differ as well, but I say - who cares?
If you want to get past your fear of public speaking then you just need to pick one and get on with practicing.
Here's how to get Impress and how to start using it.
Watch this short Impress Tutorial and you'll be on your way. (It looks best if you view it in full screen.)