Developing your site

What does everybody do when they launch a new site? I've made a few sites and although a couple of them have been quite successful I've had nothing fantastic. For me I think of a good idea that I could make my site about and then I go straight into making the layout / cms for it. I think I need to look into target audience / competition etc. more but I'm not really sure where to start when it comes to stuff like that, so what does everybody else do?