The 'film' part of the Sellotape (or at least some of it) should peel off. If you've got difficulties and it's a flat glass surface use one of those plastic thingies which has a Stanley knife blade in it, used for stripping paint. To remove the remains of the sticky bit use white spirit. (Needs a bit of rubbing to start with but then comes away easily).
Surgical Spirit (bought from chemist) should do the trick, also nail varnish remover, or perfume/ eau de cologne - they are all similar to white spirit.