Piano: Holding a longer note whilst playing shorter notes simultaneously.

Hi, I am a new user

I want to write for the piano. With the treble clef I want to write a semibrieve but I want to alsio play quavers simultaneously with the same hans, as in Beethovens Moonlight Sonata. When I try to enter the notes it always places the quavers AFTER the semibrieve. How do I get them to play simultaneously?