The elements of rhythm are indicated at the beginning of a piece of music. The time signature tells you how many beats there are to a bar of music -- 4/4 time is referred to as common time. It tells you there will be four beats of music per bar. Understanding this helps the pianist count. Counting beats is an effective way for beginning piano players to get a feel for timing. Tempo, also indicated on the music, is usually written in the upper left of the sheet music. A tempo marking might show a quarter note, eighth note or half note, followed by an equal sign and a number, such as 120. This indicates that there will be 120 beats per minute for the specified note value. This is the setting you use when you practice with a metronome.
A metronome is a tool useful for keeping time. Each tick of the metronome represents a beat. The number of ticks the metronome plays depends upon the setting. If a song indicates there are 120 beats per minute, this is the setting to use on the metronome. Listen to the metronome as you play play the song, trying to play at the appropriate tempo. Listening to a metronome early in your practice sessions can help build an internal metronome inside you. Eventually you can play without a metronome and count naturally.
Playing piano often requires the ability to handle two distinctly different rhythm parts. In some pieces, there will be a left hand rhythm pattern that differs completely with what the right hand plays. This takes coordination, which requires practice. One technique for developing independent rhythm recognition is to begin by playing simple rhythm with the left hand while executing a more complex rhythm pattern with the right hand. For instance, play four quarter note beats with the left hand while you play a syncopated rhythm of eighth notes on the right hand. Practice this at different tempos, always playing two beats on your right hand to every one beat you play with your left. When you can do this smoothly, reverse the roles of your left and right hand, then alternate their roles. This can help you develop coordination and timing required for strict rhythm control.
Pianists often play a rhythm pattern with chords and alternating bass on the left hand, and a melody pattern with the right hand built around chords as well. This can take time to perform effectively, since there are so many elements going on. Start slow with the metronome set at half the suggested tempo and play first the left hand part, then the right hand part. When these can be played at tempo, combine them and practice until you can play smoothly. Gradually work your way up to the suggested tempo of the song by increasing the metronome speed.