You press down the keys... the keys are the white and black things that should usually stand out from the rest of the piano. If it a full length piano, then there should be 88 keys all together.
Piano is played by pressing keys that correspond to specific notes. White notes are for C, D, E, F, G, A, B or Dó, Ré, Mi, Fá, Sol, Lá, Si, and black notes are for C#, D#, F#, G#, A# or Dó#,...