Tire selection and how to grip the bike?

[Tony]

How do i know which tire is for which track. Tires as in soft, intermediate, and hard. Which ones should i use on a motocross track. Another question i have is where are you supposed to grip the bike with your knees. Is it the frame or the shrouds, and when your standing up should you have your knees bent gripping the shrouds or seat or frame