Before you give your presentation to a group of people, give it a test run to a friend or SO. This lets you know how well the flow of the presentation is, plus may give ideas for improvement.
If you have never taken a public speaking class, or given presentations to large groups, the first few times you do this it may seem unnerving. As I mentioned earlier, it may be easier to start with your local LUG and get used to talking to groups of people - the local LUG will be filled with a friendly audience and you will be more at ease.
When talking to a large audience, keep the following in mind:
Don't just read the text on the slides. Anyone can put slides up and read the text on it. You are an expert!
Keep eye contact with the audience. Every now and then, look up from your screen and scan the audience. If this makes you nervous, look across the tops of the heads. From the perspective of the audience, you're looking at them. Look up, and scan from one side of the room to the other, then look back at your screen.
Feedback from microphones can cause headaches for all around. Test your microphone setup before talking.
If you do not have a microphone, try and face your audience as much as possible. Your voice will carry better.
Do not let yourself get sidetracked. Stay on the topic. If a member of the audience tries to get you off topic too far, offer to discuss it after the presentation. Others who are interested can take part then.