You can keep this dialog open while recording using RealProducer so you can adjust and test quickly.
Built-in Microphones for PC's are notoriously horrible. So a simple clip-on mic is a good investment. Make sure it is a IBM-PC Computer microphone and not a camcorder microphone. Buy it from the computer section of a computer store - Labtec is a good brand and the cost is under $10.00. Make sure the plug has a tip and two rings.
To get the level figured out, you will need to record and playback a number of test runs. generally try to make the audio sound good with the playback volume at the middle position.
Also try to make sure that you don't hit the "Red" in the audio level meter too often. Too high and things get distorted, too low and things get muddy.
Just FYI - Microphones on Macintoshes work flawlessly.
Once you have the audio level figured out - make sure to write down exactly what you did to make it work including slider position. You don't want to keep wasting 45 minutes each time you want to make a lecture re-figuring the audio subtleties.