Whether you're just learning how to play the piano yourself or introducing a young child to the instrument, songs don't get much easier than Mary Had a Little Lamb. The basic melody is a repeating pattern of only three notes that can be played with only three right fingers. Start in C major, the easiest scale to play. You can move on from there to harmonizing chords and more complex variations in different keys, played with both hands.
Steps
Method 1 of 3: Playing in C major

Step 1. Place the thumb and the first two fingers of your right hand in the C position
To play "Mary Had a Little Lamb" in C major, play the white key, aka middle C (in the middle of the keyboard), and the two keys to the right of it. These are the D and E.
- In the C position, you can play the first five notes of the C major scale: C D E F G. Your thumb is on middle C, while your little finger is playing G.
- For the basic melody in C major, these are the only notes you will use. The entire song can be played with just these three keys -- although you can make it into a more complicated melody once you've mastered the basic melody.

Step 2. Play E D C D E E E
These notes make up the first line of the song. Sing along "Mary had a little lamb" while playing. There is one note for each syllable. These notes are also the third line of the song, so once you know this line, you already know half of the first verse.
While this song can be played with just one finger, practice with three fingers so you can get used to it. If you want to add chords later or play more complex arrangements, you need this skill

Step 3. Continue with the second line by playing D D D E E E
The second line of the song is "little lamb, little lamb" and the melody is formed with the last two notes of the first line, each repeated three times. For an alternative version, you can also play D D D E G G, hitting the G key with your pinky finger.
If you've played the second line, add the third line that you already know (because it's the same as the first line)

Step 4. Play E D D E D C, the last line of the verse
The lyrics to the last line of the song are 'its fleece was white as snow.' As with the other lines, you play a note for each syllable of each word of the song.
When you get to the last line, try to play all four lines together without stopping: E D C D E E E / D D D E E E / E D C D E E E / E D D E D C. Now you can play 'Mary Had a Little Lamb'

Step 5. Repeat the same melody for the next couplets
Of course there is more than one verse in the song 'Mary Had a Little Lamb'. However, they all have the exact same melody, without any changes. So once you've learned the first verse, you can play the whole song.
- There are four stanzas in the song. You can find the full text at https://allnurseryrhymes.com/mary-had-a-little-lamb/ if you don't already know it.
- The song 'Mary Had a Little Lamb' is based on a true story of a 14-year-old American girl in the late 18th century, who one day took her lamb to school.
Method 2 of 3: Playing the melody with chords

Step 1. Learn which chords go with the notes
In each scale, each note has a corresponding harmonic chord. The chord begins with the root (the single note you would play in the melody). Then add two notes above that note and play every other key.
- For example, the harmonizing chord for C is the C major chord, formed with the notes C, E, and G.
- If you're teaching piano to a small child, this is a great opportunity to introduce some music theory into your lesson in a practical way that is likely to be picked up relatively quickly.

Step 2. Form a chord with your hands
To play chords instead of single notes, just move your hand up and down on the keyboard and play three keys at once instead of just one. Keep your hands in the same position while playing.
- Start with C major so you don't have to use black keys. You may need to practice before you can smoothly harmonize the melody in other scales, where you also have to play black keys.
- Keep your wrists loose and your fingers slightly bent in the same shape. Your fingers should not be too stiff or too clenched.

Step 3. Move your whole hand while playing the melody
To play a harmonized melody, simply play the harmonized chord instead of the single note. When playing this way, your thumb will always strike the root of the chord -- the single note you would play in the melody.
When you're just starting out, you can try playing the entire melody using just your thumb (without using the other fingers). This will get you used to moving your hand up and down on the keyboard as you play the melody
Method 3 of 3: Trying variations on the same tune

Step 1. Slide your hand to play the song in G major
If you want to play 'Mary Had a Little Lamb' in G major, all you need to do is slide your hand across the keyboard until your thumb rests on the G key (where your pinky was when you played in C).
While you should technically also use the black keys to play in G major, they are not necessary for this base melody. As long as you play the basic melody of the song, you use the same pattern as in C major

Step 2. Strike the F sharp when playing the song in D major
Move your hand so that your thumb is on the D to play in D major. When you play the first five notes of the D major scale, you play F-sharp instead of F -- the black key to the right of F. Play those five notes a few times until you get used to playing the black key.
- Since the pattern starts with your middle finger, the first note of the melody is F sharp. From there, just follow the same fingering pattern.
- Write down the names of the notes as you switch keys to learn how to transpose songs to different keys.

Step 3. Try playing the melody in A major
As in the other keys, move your thumb to the A key. You play the rest of the first five notes of the A major scale with four fingers. One of those notes is C sharp, the black key to the right of middle C.
As with D major, because the pattern starts with your middle finger, the song starts on the black key. Furthermore, the song follows the same fingering as in C major

Step 4. Accompany the melody with chords with your left hand
Use harmonizing chords with your left hand to add depth to the song by playing the harmonizing chord while playing the melody with your right hand.
- When you play in C major, you alternate between the C major and G major chords. To play the G major chord, move your hand four keys (or steps) down from the C major chord. Then just move your hand back and forth while playing the song.
- On sheet music you see the chord above the letter. For a song like "Mary Had a Little Lamb," you would traditionally play the harmonizing chord with the first note of each bar. Add chords where you want to emphasize the notes, and omit the chord for softer parts of the song that you want to play more subtly.
- Based on the lyrics, you could add a harmonizing chord to the uppercase syllables: 'MAR-y had a LIT-tle lamb, LIT-tle lamb, LIT-tle lamb, MAR-y had a little lamb, its FLEECE was white as SNOW.'