In certain styles of music (e.g. lead sheets) it may be necessary to hide a key signature after the first system. Normally you can achieve this by using a staff type that doesn't display key signatures, but this has the disadvantage that any sharpened or flattened notes appear with their accidentals on each occurrence.
This may not be what you want; if you want the key signature to be hidden but the music to behave as if it were visible, you can follow this procedure:
1. After you have created your music, create a staff type change beginning somewhere after the first bar: selecting any bar on the first system and choose Create > Staff Type Change > Pitched > 5 lines (no key signature).
2. Select all of the music starting from the bar in which the staff type change occurs. (Use Shift-click to select a passage; see Selections and passages in your User Guide for details.)
3. Press 8 on the main keyboard (not the numeric keypad) to create an octave line above your current music.
4. Filter your newly-created octave by going to Edit > Filter > Top Note.
5. In the General panel of Properties, change the selection to voice 2, and select Hide.
6. Change voice 2 to use a silent notehead by going to the Notes panel of Properties and selecting the silent notehead type. (In scores created in Sibelius 2.x, it is the tenth in the list, two after the invisible one.)
7. Now drop it down an octave by typing Ctrl + down arrow.