What is the word for the numbers at the beginning which tell you how many beats are in each bar?

The top number represents the number of beats in a bar in the piece. The bottom number represents the kind of note gets one beat. If the bottom number is 4, it is a quarter note. If it is an 8, it is an eighth note. just put one over the number and that is it. 4 for the top note and 4 for the bottom means that there are four quarter notes in a measure. 12 on the top and 8 on the bottom means there are 12 eighth notes in the bar. Hope that helps. :)

P.S. These numbers are called the "Time Signature"