Planning is an important part of effective public speaking. First, identify the occasion. A speech addressing industry professional at a convention will not be the same as a speech in front of a small, informal gathering.
In order to keep your audience's attention, you need to get to know them and understand what's important to them. This will make it easier for you to determine the best way to communicate your message to them. Get started by paying attention to the demographic. For example, are they young or old? Are they high school graduates or do they hold advance degrees?
Lead in to your speech with an attention-getting comment or question. From there, speak in your audience's language to keep them engaged. Keep the topic relevant to issues they deal with. Use examples and anecdotes to help get your message across.
Practice your speech in front of someone else several times before actually delivering it to your audience. This will help you stay within the designated time limit (ideally, most speeches should be around 20 minutes, although keynote speeches may run longer).
As you practice, pay attention to the length of the speech, your tone of voice (do not deliver the speech in a monotone), your posture (don't slouch) and gestures (a little goes a long way--otherwise, gestures may become distracting). It's always a good idea to record yourself, if possible, so that you can both see and hear how you're doing.