If the police have contacted you, or stopped you on the street, it is because they believe you have committed an offense and intend to arrest and charge you with something. It is not a social call.
Be polite to the police. Angering them will not help your situation and often results in their finding additional reasons to charge you and additional charges.
You are entitled to ask them why they are stopping you. They must have "reasonable and probable grounds." However, cooperate with them at this stage. Do not play lawyer, or be rude or insulting. But be very discreet in answering their questions. Do not lie, but if you do not wish to answer a question, just say so.
You will not talk your way out of an arrest if they intend to charge you.