Like everyone here has said, use tables and inline all your css... but there is an ecosystem of email apps to help you build emails.
I've been using Mailrox (https://www.mailrox.com/) for most of my email builds recently and it seem to be pretty damm good and outputting perfect HTML emails, if you're building one from a design, even though it's in beta.
You could also try pre-built templates from Mailchimp or Campaign Monitor, but it sounds like you have a design for your email so maybe Mailrox would be best.
If you really want to get into building emails I'd say forget most of what you know about modern webdesign and master table layouts and use the links from PatrikAkerstrand.
Litmus is also great for testing your hand-coded designs. They give you previews of your email in (pretty much) all the email clients.
Hope this helps.