Where should I place my lips on the mouthpiece to get the best possible sound?
There have been many attempts to make rules for this. One is the 2/3 rule. Another says that the mouthpiece should be exactly on the middle of the lips horizontally. I do not believe fully in any of them. I have seen people successfully play with the mouthpiece all over the lips. Just make sure it does not hurt and that you get a good vibration/buzz. Pressing too hard can cause less free vibrations. Also keep in mind that the inner part of the lips vibrate more efficiently than the dry outer part.