That is something you will gain by practice. You need to be confident and talk in English.
There are a lot of things to look for. You need to pay attention to what a person is saying. You should never disagree by directly saying, 'NO'. You can do that indirectly by saying, 'I beg to differ...'.
If you want to improve English, watch movies and TV shows. To practice you can call up any place like hotels, resturants, etc and start talking. Ask them about their place, products. I know this sounds silly but I think its great to boost your confidence. We all know to what extent, 'Talk to your friends in English' works. :P
You can research on this topic in the internet and you will get a lot if content.
All the best.