As a chinese from Mainland China. I know exactly how you feel. We spent too much time on learning grammar kind of things and we screwed it up at all. From my experience, I just tried my best to express what I wanted to say and learned the way they spoke. Don't be shy, take your time. They know English is not your native language and they would listen to you patiently.

Being nervous is normal. Especially, when you start to talk in a language you are learning. You can overcome this feeling through practicing.

About what subject to talk, infact it's not a matter of language. It depends on the people you meet too. Even in your own language, you might not find a subject to talk to with some people. Talking in a foreign language even eases talking since you start to get to know each other. If even this phase does not go smoothly just the person in front of you is not interested in talking. Give it up. Otherwise one subject opens another one.

Thanks for all replies...

I think you could ignore all the stress and the pressure and just have fun when speaking.

If you're uncertain about what to talk about you can ask your partner/teacher to choose a topic for you.

As long as you don't worry too much about doing errors and stay enthusiastic I'm sure you will find someone who can really listen to you and help you to improve your speaking :) 

У меня тоже самое когда начинаю говорить теряются слова и не могу сказать и забываю слова английские.