From reading your questions and answers so far, you are a long way from really knowing how to take advantage of Lightroom and before you jump in feet first, it's optimal to learn about what you are planning on doing. The fact is, starting LR only with newly shot images isn't what LR is all about.
Though the kind of questions you're asking are the right ones. I don't think it's a bad idea to start off with new images, but pretty soon I think you're going to be at the point where you do understand what your catalogue is and will then want to bring all your pictures under Lightroom's control.