The key you are playing in

The key you are playing in is the set of notes a piece lives in, written as the sharps or flats after the clef. C major's key signature is empty: every note is a white key.

Twinkle, Twinkle, Little Star lives in C major. The third phrase uses the same white keys you already found from the black-key groups.

Key signatures are the sharps or flats printed at the start of every staff, right after the clef. Each one applies to that note in every octave for the whole piece, unless an accidental in the bar cancels it. One sharp on the F line means every F is F♯.

They are also a shortcut, for the hands and for the ear:

• The fingers know which black keys are coming, so the scale and chord shapes of the key are ready before the notes are read.
• The ear learns the "home" feeling of the key, and reading sheet music gets easier because the notes lean somewhere you can hear.

Before playing, look at the key signature and find the tonic (the "home note") on the keyboard. That one glance orients the whole piece. In C major the tonic is C, and the C major scale is the set of white keys from that C to the next.

The third phrase of Twinkle, Twinkle, Little Star. Still C major, still white keys, now with the key signature in view.