Look at the time signature for your sheet music to determine how many and what type of notes are assigned to each bar. The top number indicates how many notes are in each bar, while the bottom number indicates which type of note is counted. For instance, if the top number is four, there are four notes per bar. If the bottom number is also four, that means that there are four quarter notes per bar.
Check for tempo markings on the music so that you know how fast it goes. A piece marked "adagio" will be slow while a piece marked "presto" is very fast.
Set your metronome to reflect the tempo markings on the piece. Choose to set it so that it clicks, blinks or both.
Clap the rhythm on your sheet music so that you become accustomed to the rhythmic sound of the piece. Each printed note on the page gets its own clap and the group of notes between the bar lines equal one bar.
Count out loud while you are clapping and stay in sync with the metronome. If there are four quarter notes to each bar, you will count to four over and over while you are clapping the printed rhythm, moving from bar to bar. This repetitive counting is what musicians mean when they talk about counting bars.
Add the rap lyrics to the piece while you are clapping, continuing to count the rhythm in each bar silently. This works best if you have memorized the rap lyrics in advance.
Tap your foot to the beat instead of clapping and continue rapping the lyrics. Stop tapping your foot once you are comfortable with the rhythm of the piece and are easily able to count silently while rapping.