Depends on how soon you plan on selling I think. If you want to get the business going, I say just start one. There's going to be plenty to learn along the way and you can at least get a jump on some of it. Switching to a new theme is as simple as pushing the publish button and reconfiguring your stuff in the customize editor. If you just want to get your toes wet start a Shopify Partner account and make a test store. You can keep them indefinitely and use it as a little playground to get to know the ins and outs of Shopify.