We are all terrified of speaking in public. Every one of us. I’m a performer and sometimes even a wedding MC, and I still get nervous. The only way to feel more comfortable about speaking in public is to do it well, and more often. As you build up memories of positive experiences, of of trying new things, you’ll quickly start feeling (and looking) relaxed and capable when you speak to groups of people.
Here are some tips to help you give great presentations:
Have a clear objective: Decide why you’re speaking. You should always have an active, clear goal for your speech. To ‘inform’ people is not active enough. You want people to sit, to listen, and eventually to do something. Knowing your objective will tell you what needs to be said, and what other bits can be left out. If you don’t have one, decide now.
Structure your presentation. If you can, rehearse it, but sometimes you may be called upon and you won’t have time - so get up and do it, just follow these three steps:
●    Attention: Start out strong. Get their attention. Remember, your audience will have decided a lot about your credibility before you’ve even spoken; and not just from what you say, but also in how you say it and hold yourself. Walk confidently to the place from which you’ll be speaking, stand for a moment and connect with your audience, and then project your voice clearly. Get their attention. Begin with an amazing fact, a bold claim, or a riddle: “Why is legislation like my mother-in-law?”. 
Your attention-grabbing start could also be a matter of stating your objective: “At the end of this talk, I am going to ask you to fill in a form to become an organ donor”. It tells us why we’re listening.
●    Emotion: Change how they feel. Too many speakers rely on statistics and facts to fluff out their presentations because they’re easy to google (and regardless how crazy your proposition, someone, somewhere will have compiled some statistics that make it look reasonable). Please, don’t. Forget facts, unless they’re amazing and emotive and even then, used sparingly.
Tell stories. Use examples. Appeal to people’s greater virtues. Move them. Make them want to agree with you. Inspire useful emotions in them. If you want them to stop doing something, use negative emotions like sadness and guilt. If you want them to do something, or change in some way, you must aim for positive, energetic emotions like excitement.
●    Action: Get them to do something. Ask them to fill in a form, send you an email, or even just raise their hand. Ideally, get the listener to express their support for your cause, and take some small step towards it. Once they feel that they’re already started, they’ll be much more likely to follow up with larger commitments of time and effort; and once convinced and started on it, they will actively try to persuade others.
Other presentation points:
Don’t use PowerPoint as a crutch. I’d actually prefer to tell you not to use it at all, but sometimes that idea freaks people out so badly they end up whimpering in a corner, cuddling a USB key. So fine, if you must, use a few slides - but use as few as possible. Don’t write your speech on them. If they’re so extensive that when someone misses your presentation you can say “I’ll send you the slides”, then there probably wasn’t a need to give the presentation in the first place. 
As a guide:
●    No more than 1 slide for 2 minutes of speaking
●    Only essential slides: If you deleted any one of them, the presentation wouldn’t make sense
●    Nothing that appears on the slides could have been adequately explained verbally (pie charts are okay, clip art is not)
Do it more. Take as many opportunities as you can to speak, with as little time in between as possible. As your body gets used to the experience of standing up in front of crowds, your stress hormone levels will normalise, and you’ll start to feel more relaxed. Focus on letting yourself be yourself. It’ll feel more familiar and give you access to your sense of humour, making you more friendly and relatable.
Do it differently. Try doing your presentation a bunch of different ways, particularly if you’re giving the same talk over and over again. It’ll keep things alive in your mind, and it will give you a richer arsenal of experiences to draw upon.