I'm wondering if it is Sellotape or Cellotape, or anything else(Sello tape or cello tape). I'm talking about the general name of that roll of sticking tape.
The brand name is Sellotape. It is commonly written sellotape in places where the name has become genericized (i.e., is no longer an exclusive trademark).
Note that not all parts of the English-speaking world use or understand this word; here in the U.S., we say scotch tape (which also originated as a brand name). In fact, I had never heard "sellotape" until a few years ago, when an Indian coworker used it, and I had to ask her what she meant by it. (That's also when I realized that the Spello-tape brand in the Harry Potter series of novels is a pun.)