There are two main ways to create an email in Marketo. Let's look at both.
Create an Email in the Design Studio
Go to the Design Studio.
Click the New drop-down and select New Email.
Create an Email in Marketing Activities
Go to Marketing Activities.
Select the program you want to add the email to, click the New drop-down and select New Local Asset.
And that's it!
Whichever method you choose brings you to the template picker.
Give your email a name, click on the template you want to use, then click Create.
Enter a subject line. There's a counter on the right to let you know if you're under the recommended 50 character max.
Depending on the template you've chosen, you'll have different options in which to edit your email. For emails with modules, check out Add Modules to your Email.
Your email is now created, so edit away!