I'm not sure what your question is. Are you asking how to refuse in English? Or how to refuse in general? If it is in English, we could perhaps translate from what you would say in Chinese. In general, that depends on the situation, and on you. 'Some friend wanted me to help them do something,but I think I can't do it .and I don't know how to refuse". Remember - capital I. And 'them' when they are the object.

how do you say refuse? Move the head from side to side while saying:- "I'm sorry but I can't do that." "I'm afraid I can't help you." "Sorry, but no way." -- informal "I am unable to do this". Is an explanation required? "I'm busy with other things right now."