The first thing you need do is register for a Shopify Partners account. After you have accomplished that, you will have the ability to establish what are known as "Development stores." These stores are EXACTLY the same as standard stores, except that they are free permanently. However, once you've reached 50 sales on it, you will need to begin making payments. However, by that time, you will have made 10 times that monthly plan payment regardless, so it doesn't really matter which one you choose.