SPEAKING PRACTICE

Last week I was planing to attend a discussion class in the afternoon, but, in fact, was not what I was waiting for. We went to a language laboratory in the university and then, the teacher (I don't remember his name..), told us that we were going to prepare a presentation about "acid rain". We had 25 minutes to prepare our speech using some audio/video and written documents. The teacher came back and we had to expose our presentation recording it on a small camera. It has been a complete desaster for me! While I was reading the document and whatching the video, I was thinking that was not so difficult: I already knew the meaning of most of the words used and I also knew,more or less, the acid rain process...but when I turned on my camera, after maybe the first two sentences, I get completely lost even if I had my notes in front of me. What does it mean? I think that in my head, in my mind I know the meaning of lots of words in English, so either when I hear or read them I don't find any difficult to understand, but when I have to build a sentence those words don't came out! What can I do? I have to put more attencion in reading and listening, trying to "keep" what I know already. In addition I should speack more English, about everything, to make the passage from thinking to speacking more fluid.