depends strongly on what you want to do. you can't just say "use oscommerce and that's it". if you want to build a professional webshop and live from it someday you should think more complex from the beginning.... if you make mistakes in the beginning you can have big - and expensive - problems later... (trust me i know what i'm talking about...)
1) go to a professional / experienced web-designer and get a layout. not a oldschool-webdesigner, but one that knows about stuff like web-usability/split-testing/...
2) shop software. most will say go for os/xt-commerce or magento. i'd never use those tools for a professional shop (i'm much to afraid of exploits/securityholes/... to use it. if the shop client data go into the wrong hands i'd be really fucked). it's a good low-cost start maybe - but if you scale quickly you have to think about moving to something different - and there we maybe have a data migration problem... so choose wisely (one of the reasons why we delevoped our own shop software for our clients. you're much more flexible there... oh and try to organize 300.000 articles in open source software).
3) ERP software... this is also very important... there are hundreds out there. the software should have connectors for importing/exporting all data (esp. clients/orders) and features for organizing your business. take a look at eg. "microsoft dynamics NAV" (at least the website) to get an idea
4) marketing... the expensive part... online/offline marketing... this will cost the most money...
if you have any specific questions just ask me. i built up several shops for clients so maybe i can give you some infos