This is an online piano keyboard with three octaves that can be played on your computer.The Middle C key is the second C key from the left, it's the one with the red letter.
You can play the keys by tapping the letters and numbers on your keyboard. The Middle C key, for example, can be played by tapping the "u" key. The keyboard equivalents are labeled on the piano keys.
You also can use the mouse and click on a key.
Just make sure you have the volume turned on!
This virtual piano keyboard shows three octaves - octaves 3, 4 and 5. The first set of keys, which include the first seven white keys, are octave 3. The Middle C key is the beginning of octave 4. And the right-most set of keys, on this keyboard, are octave 5. The very last key on the right is an extra C key, C6, which is the start of octave 6.
A lot of songs use these keys, especially beginner student-level songs.