It's quite simple.
1st - you need to decide what platform do you want to build your shop on (i.e. magento, opencart, presteashop etc. - just google it and pick the one you find best fit for your needs) and where do you want to host it
2nd - make your own theme (or buy it - usually there is quite a theme market coming with each platform)
3rd - get your logo done
4th - get your supply chain sorted
5th - connect card gateway (and/or paypal) to your platform (there are many mods available, some are free and some are paid)
6th - you are on-line, from now on you focus on tweaking (branding, SEO etc.)
Good luck!