When you have to speak about a topic, it is generally expected you fulfill certain communicative aims and you perform certain language functions, too. Have a look at this and see what I mean: http://www.talkingpeople.net/tp/skills/speaking/brainstormingexample.htm
You will find more ideas for working on your Speaking here: http://www.talkingpeople.net/tp/skills/speaking.htm
But remember: listening to English is key. When you listen to English, you learn to speak, you consequently learn “grammar”, and you get used to understanding people, while developing comprehension strategies unconsciously too! http://www.talkingpeople.net/tp/skills/listening.htm