When it comes to website building and promotion, there seem to be two general approaches you can take. One way is to be methodical and detached. You spend hours and hours doing lots of keyword research, exhaustively sussing out your competition. Using this detailed information you choose a traffic pulling domain name, then carefully optimize each of your pages. Building the site itself and then gaining backlinks to it is almost a formality.
The other way is to depend much more on intuition. You just get an idea for a site, then throw down some relevant content as soon as possible. You then let it develop organically, checking your stats as you go, seeing where your traffic is coming from and then building on that. With this approach you are basically making it up as you go along.
So which is the best approach? Of course there are many who would who say the first one is vastly superior. You may spend many hours in preparation, and comparatively little in execution. But this will be worth it in the long run when you are pulling in solid targeted traffic for years on end without having to do much more work.
I prefer the second approach, however. While I do think it's a good idea to give a lot of thought to what you're going to focus on - and you should certainly do some keyword research in choosing a domain name (unless you are using it primarily for branding, as opposed to SEO) - I think you can easily overdo the preparation.
Why? Because the fun of website building is in sharing the information. Just having some good useful content up there is a great feeling. Okay, so it's not optimized yet. But you can always do that later. The search engines will index the changes when completed and you will have done no permanent harm to your site's potential in the long run. It's not set for all time once you press "publish"; it's dynamic and alive and always a work in progress.
The other reason is that you can't predict what your competitors will do. You might spend weeks finding low competition, high traffic keywords to focus on and thereby isolate a juicy little traffic vein. But in a few months there may be six other webmasters who are also onto it, reducing your flood of visitors to a trickle. There's also the possibility that the search engine algorithms change to your disadvantage.
If both, or even just one, of these things were to occur your site's rankings could take quite a hit, and you'd feel very much like you'd gone right back to square one. But if you use the more intuitive and improvisational approach you can take these changes much more in your stride and keep developing your site with enthusiasm.