Think in English. It may be difficult at first, but with the passing time you will see that it gets easier. Try to say quitly, in your head, to yourself what you see before you, what the weather is, how is somebody dressed, etc. A good idea might be also to imagine a conversation with somebody and to take part in it in your head. In such situation you gain confidence in your verbal skills. As a result, you are less stressed when you speak for real.