Ask HN: How do you build a service?

I have an idea for an eCommerce web app that I want to launch either late this year or early next year (I'll be testing and such before then). There will be a self-hosted solution for open-source lovers, and a hosted service for people who don't want to deal with setup and maintenance.

My question is this: How do I actually get started with the hosted service? I've never done anything like this, so some pointers would be much appreciated. I plan to use DigitalOcean because I love their offerings.

Actually, one more thing! Every store would be a sub-domain, like how accounts are on deviantART. Need to figure out how to make that happen automatically via sign-up though.