There are different ways to code your email template:
- Code from scratch, writing all of your CSS directly inline - gives you a better handle of your code and makes it easy to troubleshoot later
- Write CSS in head or an external file and use an inliner tool - makes writing CSS easier and allows for more complex styling
- Use a framework/system that compiles your HTML and CSS - provides better management of more complex template building
Choose which way you're most comfortable with as that will become the most efficient way to do it over time. Learn by example by looking at other people's code. Take advantage of libraries, template systems and frameworks.