first store I highly suggest you start by only shipping to the US. Don't worry about international shipping until you get a request. Remember: it's much easier to build something simple today and update it tomorrow than to plan the perfect e-commerce site.
Twenty Twelve when building your site. Some themes override core WooCommerce functionality incorrectly and break things. It's best to use a simple theme for testing and then switch to a fancy theme once you're done.
products (think of a t- shirt with multiple sizes). Since there are so many options I'm going to cover it at the end. In case we don't get to it here's an excellent video tutorial: http://docs.woothemes.com/document/ product-variations/
traditional brick and mortar stores use to handle credit card transactions. Their POS systems validate the buyers card and the merchant account draws the funds. The funds sit there for a day or two and then they're deposited in to the bank account.
so many e-commerce stores that don't need a merchant account there are a lot of services that combine the merchant account and the payment gateway together. So they validate and draw the funds. These are called modern payment gateways.
add a tax rate for your city. Make sure to set different priorities. Only one line item per rate per priority will be matched. More info: http://docs.woothemes.com/document/ setting-up-taxes-in-woocommerce/