What should I set up before I launch my store?

Work through these in order:

1. Products: add them with prices, stock and images.
2. Payments: enable at least one gateway or Cash on Delivery (Settings → Store Settings → Payment Gateway).
3. Shipping: manual shipping, zones or a carrier (Shipping Setup).
4. Tax and store details: tax rates, site title, support email and phone, logo and store address.
5. Theme: choose a theme and arrange the home page.
6. Click Go live in the top bar.

Then place one real test order yourself, from the product page to the confirmation, before you announce the store.