If you want to talk like a native speaker then you have a long way to go. You can't skip over the steps. I suggest you master the basics first, before moving on to idioms and expressions. They're forums over here that can help you starting off(Vocabulary and Idioms, General English garmmar questions, and Audio: Speech and Pronunciation). You'll find them on the home page.
About your voice, just be yourself, do not force anything. It's sometime difficult for the non-native speakers to change their accent, and accomodate one(when you can get one, of course) because, there are different accents. American, British, Canadian,.and so on. Generally non-native speakers are confused on that issue. (There is a thread going on) I'm definetely not the right person to tell you what accent is the best. I have a....Cameroonian accent I must say. Anyway, my point is, first make sure you can not only make yourself understand, but also express yourself as clear and good as possible, then you can go accent-hunting.
About the topic, It surely depends on your interlocutor, the occasion/environment, and may be his gender as well. You can always take Englishforums as the environment, and our discussion as the content. But what would YOU want to talk about?