The first knob sets the key. Key is only necessary when you are trying to sing against a song and you want your heavily pitch corrected voice to fall on certain notes that sound good against a particular song. If you know nothing about music, or if you are only jamming with yourself, don't worry about this knob and just leave it at C. Otherwise you'll need to find out what key the music you're singing with was written in.
The second knob sets the amount of "Hard Tune". If set fully right, you'll sound like T-Pain or any of the billboard artists who use heavy pitch correction in their songs. If you set it closer to off, you'll get subtle pitch correction that will help you sing more in tune without your audience noticing that you're receiving a little help from technology.
The third knob is "Gender" and can make you sound more feminine or more masculine. Sounding like a chipmunk or an evil masked space villain is not out of the question.