The best way to improve your public speaking abilities is to practice. The more often you speak in public, the more confident you are likely to be the next time you have to do it.
There are, however, no guarantees that the nervousness you feel before speaking or performing in public will ever go, although you will definitely improve. You could also practice in front of a mirror, again and again. Don't try and speak impromptu, unless you have done so before and are skilled at it. Make sure you are well prepared with what you have to say, so you don't stumble too much. Churchill, who was an excellent orator, would practice his speeches for hours in front of a mirror - and would rehearse every single pause, and every single time he would stop and take a sip of water. Practicing in front of a mirror will make you more confident as to the contents of your speech, and that will be one less thing for you to worry about. However, this will not take care of the core issue of nervousness when you are facing an audience. The only thing that can help with this is practice in front of an audience.
Understand and accept that it is perfectly normal to feel nervous before making a speech in public. Almost everyone feels nervous - some less than others, but they have conquered their fears and are now out there, giving it their best shot.
Where will you be making your speech or giving your presentation. Visit the venue in advance if possible, so you can familiarize yourself with the room. Seeing the size of the room will help you be mentally prepared and aware of the number of people that will be present. After all, if you expect to speak in front of 20 people, and suddenly find yourself facing an audience of 500, it can be extremely nerve-racking. You should therefore also try to find out the estimated number of people expected to attend the talk.
Realise that everyone in the audience are on your side. That's right. If you stumble, they would all, consciously or sub-consciously, hope for you to recover your wits and continue with your talk.
No matter what your talk is about, try and inject a bit of humour in it. You don't need to worry about whether or not your jokes are very funny, unless you are a stand-up comedian. Even slightly witty or funny statements provoke reactions and smiles in the audience, which is what you want.
Think about the fact that giving a speech should technically be really very easy. Stupendous performances are not expected. It would be nice if the speech is entertaining, but this also not expected. In fact, many speeches tend to be a little boring, and audiences expect this. So a little entertainment is also greatly appreciated. You can also refer to notes written to yourself, or you can write the entire speech and keep referring to sections. There are no rules, except for the fact that you need to keep yourself from losing your nerve. And in the case of public speaking more than anything else, there is nothing to fear but fear itself. Public Speaking