Whenever I'm in front of a crowd, my hands start trembling and my knees begin shaking. What can I do to prevent this?
Performance anxiety is among the most common of human fears. The first steps to trying to deal with this involve thoughts (cognitions) and behaviors (avoidance).
In respect to the former, try to write out your fears, question them, and write down some helpful thoughts to remind yourself of before your next presentation. Helpful thoughts tend to be focused on the task at hand (rather than your fears), and should be realistic and under your own control(e.g. "I have something I want to say", rather than "I hope they don't notice my hands shake").
In respect to the latter, find some opportunities to practice. You might start with the mirror and work your up to more challenging audiences. If it's hard to find a venue for practice, the self-help group Toastmasters is a great option. Hundred of chapters around the world meet periodically to practice public speaking and get helpful feedback on speaking skills. Check their website for info.
If you are unable to make progress on your own, consider a consultation with a mental health professional experienced with performance anxiety or social anxiety disorder. Main approaches are supervised cognitive behavioral therapy (similar to what is described above); and medications that can be taken as needed, or on a regular basis if necessary.