Before you even give a presentation, there are a few things you have to do:
Choose a topic.
Submit your topic to a show or LUG. (and get it accepted!)
Create a presentation from the topic.
Give the presentation.
Contrary to poular belief, each of these may be as hard as all the others. Choosing a topic for some may be easy, based on their expertise, whereas a Linux jack-of-all-trades may be stuck choosing a topic. Those comfortable with public speaking may have trouble designing the slides, and so on.
So let's jump right in and hit all the topics and get you started!