A time signature is what tells us how many beats are in a bar and which kind of note gets one beat. The top number tells us how many beats are in a bar and the bottom number tells us which kind of note gets one beat. It is at the beginning of the staff right after the clef and the key signature. The treble clef and the bass clef are always first, then the time key signature, then the time signature and then come the bars of music.
For example; a 4/4 time signature means there are for beats in a bar and the quarter note gets one beat. Another example is 3/4 time; this means that there are three notes in a bar and a quarter note gets one beat.