in demo teaching, what will be the best topic you are going teach?
It depends on the subject and the level. The basic gist is as follows:
 Ask the students what they know about the topic you are going to present; e.g., Introducing oneself.
 Go over what they know, their good and bad uses of the language.
 Add new language; e.g., words, phrases, meaning, colloquialisms.
 Have them work with it; e.g., a text that houses the new language and alternative forms so that they can see or hear how it's used by native speakers.
 Have them add to it using their knowledge about making introductions in their culture.
 Have them practice their findings.
 Show them where they were (their old knowledge) and where they are now (their new knowledge).