Is there software that I can enter (kind of type) an existing music sheet and play it on the computer choosing piano for example?
Proprietary/Commercial: Finale, Sibelius
Free: MuseScore, Lilypond
Finale can import scanned scores.
MuseScore is acceptable for small projects, but for larger ones you should probably buy software. Long run, it is worth the investment because the free programs tend to have bugs and this slows down production, at least I have found it so.