Speaking on the radio isn't exactly public speaking, since you don't need to see your audience, and it's a lot easier than speaking in front of a crowd or a teacher. Mastering it does require some experience, however. Here are some do's and don'ts for beginners.
Get someone with experience to show you how the sound board works. Every mixer has its own quirks; the more you know in advance, the better prepared you'll be when something goes wrong.
Pick out some background music that you're comfortable with. This is the easiest way to keep from panicking about dead air. With some music behind you, you needn't worry about minor pauses. Be sure and pick instrumental music; vocals are distracting.
Clear the room. The first time you speak on the radio, it should be by yourself. Other people are distracting, even for many pros.
Don't swallow the mic. Keep it at least a fist's length away from your face. Speaking with your lips on the microphone increases the chance of distortion and wind (breathing into the mic).
Speak plainly and clearly. Don't confuse the listener by dumbing down your diction or forcing an accent.
If you make a mistake, apologize briefly and move on. The listener will forget about it if you let them, and new listeners won't even know that it happened.
Keep an even tone. If you're going to bark out the set list over surf music, don't start whispering and make the audience strain to hear you. If you're going to speak quietly over Enya, don't start yelling.
Things to avoid
Try not to eat while you are on the air, especially when your mic is live.
Keep it concise. Some listeners will want more talking, and some will want more music. You should set your own clock, but bear in mind that unless you have a talk show or an interview, most listeners probably won't wait more than five minutes for the music.
Be consistent. If your show is about politics, don't rant about the film industry. If you have a talk show about sports, don't spend a lot of time criticizing the president. Straying too far afield will alienate your core listeners.
Don't invite pals. Nothing sounds more amateurish on the radio than a long and rambling conversation between casual friends, some of whom will almost certainly be inaudible to the listener.
Keep it professional. A casual, conversational tone is fine, but chances are good that strangers won't want to hear you criticize former lovers or neighbors at length. Excessive in-jokes will probably alienate new listeners.
Don't be Howard Stern. Imitators are a dime a dozen, and without his fame and reputation, you're more than likely to come off like a jerk. Develop your own voice and leave Howard to his.
Be prepared and have extra music to play. Always keep an eye on the clock. Be sure to fulfill all your legal and advertising requirements; this should be top priority.
Be on time; don't force the DJ ahead of you to stay late. Don't leave a mess for the next DJ and don't keep her waiting. Don't show up drunk or stoned; the chances of making mistakes increases exponentially, and might cost you your show.