(i) Almost always pronounced like sh in ship. Examples are: fisc fish, disc dish, scort short, scinan to shine.
Note: To show this quality of sc, the Anglo-Saxons often inserted e between the sc and a following a or o. Examples are: sc(e)acan to shake, bisc(e)op bishop, sc(e)olde should.
(ii) Occasionally pronounced as sk in skin. Examples are: ascian to ask, Scottas Irishmen/Scots.
It looks as if the sound sh ocurred before e and i, and not before a, o and u. The same rule is valid for sk in Scandinavian languages (not in Danish). Most probably, however, it was sk in all positions in an older period.