Purchase a beginner's book on music notation. Until you get used to the note rhythms and how they are counted, you'll want a reference handy. You can find this information online free of charge. Start by getting acquainted with the time signature, located at the beginning of a piece of sheet music. The time signature is written like this: 4/4. This means there are four quarter note beats per bar of music. 4/4 is the most frequently used time signature, so it is also referred to as common time.
Get familiar with the basic note durations you'll need to know to place the proper beats in your bar. These note durations include whole notes, half notes, quarter notes and eighth notes. A whole note lasts for the entire four beats in 4/4 time, which means a whole note is played and held out for the duration of a bar of music. A half note takes up two beats, which means you can fit two half notes of music into a bar in 4/4 time. Continue to break this down and you can see there are four quarter notes of music per bar in 4/4 time and eight eighth notes. A bar of music in 4/4 time has to equal a total of four beats using any combination of these note values.
Set a metronome to about 60 beats per minute. This is a reasonable pace for beginners. You can set more beats per minute as you become accustomed to counting. Now, for practice, count four bars of music, each containing a whole note. Listen to your metronome and when you're ready to count, snap your fingers on the click of the metronome and count one, two, three, four. Do this several times to get the count of a whole note, only snapping your fingers on the count of one each time. Now do the same thing for half notes, only now you will snap your fingers on the count of one and the count of three, allowing each snap to last for two beats.
Count quarter notes by snapping your fingers on each count. There will be four finger snaps per bar of music at one, two, three, four, then you will start on one again for the next bar of music. For eighth notes, snap your fingers twice on each count; the count of one will have two finger snap, two will have two finger snaps, three will have two finger snaps, and four will have two finger snaps, then you'll start on one again for the next measure.
Practice combining these notes in measures when you feel confident counting them. Be sure that each bar of music only has four beats, regardless of the combination of notes you use. The more you practice counting, the easier it will be to recognize the beats. Always use your metronome and it will eventually give you a built-in rhythm of your own.