podcasting

After selecting a track to record narration by clicking the track name or track icon, choose the sound input source for that track. 

Look at the Track Info pane at the bottom right of the GarageBand window. If you don't see the Track Info pane, select your track and then choose Track>Show Track Info from the GarageBand menu at the top of the screen. You can also press the small i button as shown below.

With the the Real Instrument highlighted, click the Input Source option menu. Select your audio input—the Logitech USB Headset in this example. For simple narration from a single monaural mic input, choose Mono 1 rather than the default stereo option.

Below the Input Source selection are the Monitor options. Choose On with Feedback Protection. This allows you to hear your voice while you’re speaking. The sound is played with a split-second delay. If hearing your voice in the headphones with a split-second delay is too distracting, choose Monitor>Off—but be sure to keep an eye on the sound level of the track.

The “feedback protection” will pause your recording if GarageBand detects feedback from your headphones. If the feedback protection is too sensitive, your recording will repeatedly pause. Turn down your headphone volume using the standard Macintosh volume controls. If feedback protection is still is too sensitive, choose Monitor>On—but be sure your headphones aren't so loud that the mic picks up the playback from your headphones.

Levels

Click the track name or track icon to select a track. Make sure the record button in the Track head is red (if the record button is not red, click the button). Speak into the microphone. The sound levels (simply called "levels") move as you speak. The green level bars will move to the right as you speak louder.

Ideally, when you record, the levels will consistently register to the right of the midpoint but before the red volume indicator. When speaking in a normal voice, your sound levels should register in the green section about two-thirds of the way to the red. If the sound levels reach the red indicator, the sound volume is too high (or too “hot” as audio professionals say) and sound clipping will occur. Sound clipping makes that portion of the audio unusable.

Setting sound input correctly is extremely important. A typical problem is recording volume too low when speaking in a normal voice. If the sound is too soft listeners will struggle to hear, and you will lose them. Adjusting the input volume may require returning to the "Sound" System Preference.