From advice I've been given on the forums and other sources around the internet, it seems that to start it's better to buy the components you need to get something moving, lay them out and then work out the design. This doesn't seem to exclude having an initial idea about what you want to build though. In my case I wanted to start with a wedge/pyramid shape robot but in the heavyweight class. Now I'm going to do featherweight on the advice of the experts here who've done it a lot longer than both me or you have.

My point of view, even before I thought about weight classes and what components to buy, is that I want to keep to the KISS principle (Keep It Simple, Stupid). It works in my line of work as a developer, and can apply to anything. I don't think anyone here or in the tournaments will look down on any new comer like you or myself if we brought something simple to the event for our first time. At least then with a simple robot it would be more reliable, and we'd have more fun battling others.