Reading the keys
Every white key is labelled with its note letter. The letters run C D E F G A B from left to right and then start again at the next C, one octave higher. The easiest landmark is the group of two black keys: C is the white key just left of it, and D and E sit around it. The group of three black keys is surrounded by F, G, A and B.
The black keys show sharps: C♯, D♯, F♯, G♯ and A♯. Each also has a flat name taken from the key above it (C♯ is D♭, A♯ is B♭), so letter music that uses flats is played on the same black keys. Middle C carries a small dot.
Playing from letter notation
Songs written as letters, such as E D C D E E E for the start of Mary Had a Little Lamb, can be played straight off the labels. Click or tap the keys, or press the computer keys, which play the same notes: with the default layout the bottom letter row starts on C3 and the top row on middle C. Keep Space pressed and every note rings on until you let go.
For that nursery tune written out in computer-key letters, with the keyboard pointing to each next note, open Mary Had a Little Lamb.
Practising note names
Press Record, play a scale from C to C, and play it back: the keys light up as they sound, which helps you connect the letter with where it lives on the keyboard. When you know the white keys, switch Labels to No labels and try to find each note without help, then switch back to check.
To learn with the octave number too (C4, D4…), choose Notes with octave in the Labels menu; for Do Re Mi names, use the do-re-mi piano.