On a clock with two hands, the minute hand is the longer of the two (the shorter one being the hour hand). On a clock with three hands, the minute hand is usually the one in the middle, size-wise (the biggest being the second hand).
When the minute hand points at the 12, at the very top of the circle, the hour hand is pointing directly at one of the other numbers and it is the beginning of that hour. It makes a full revolution in 60 minutes (moving clockwise unless you have a Bavarian clock), during which time the hour hand moves steadily from one number to the next. The top of the hour is when the minute hand is pointing up, and the bottom of the hour is when the minute hand is pointing down.
I've always found it odd that the minute hand should be longer than the hour hand, when the hour hand is more significant than the minute hand.