I don't have Sibelius here to check, but you should be able to create a standard piano staff by selecting the Piano template when you create a new file. In an existing file maybe you could change the staff from a single staff to a piano staff -- in the manual there seems to be some discussion of this in the section about the Home tab (changing Instruments?). Every score program I've used has a way to set the dividing note between piano staffs and even to manually move notes between treble & bass clefs, and I'm sure this can be done in Sibelius as well, but again, I don't have Sibelius here so I can't be more specific as to the exact procedures for any of these settings and adjustments.
I've attached a link to a Sibelius course (above, click More Info) specifically about creating a piano score -- perhaps that will provide some guidance. There's also a more general beginner's Quick Look course that might be helpful..
Btw, I've moved this thread to the Sibelius subforum (Forums Home > Audio > Sibelius).