If you are asked to give a presentation, try to choose something that interests you. You need a topic that you are very familiar with and can research quickly and efficiently.
When you have decided on your topic and are writing about it, make some "cue cards" with the important points written on them.
Make the presentation eye-catching and attention grabbing. Get a couple of pictures or illustrations to accompany it (a picture paints a thousand words, but don't overdo it).
Rehearse your talk in front of a mirror, try recording it (or even videoing it) and play it back. If you're feeling really brave, post it on YouTube and invite feedback, but be warned, some people on the net aren't very nice.
It may also help your confidence to give the presentation in front of supportive members of the family or your friends.
These tips should make sure you are confident by the time you give your presentation.