First get acquainted with the basics. Make sure that coding is fun and interesting. I recommend Codecademy for a free and fun introduction to programming.
After having gained some familiarity with the fundamentals of HTML, CSS, and JavaScript, decide if you want to build your portfolio by following a structured path or a more creative path.
If structure is what you crave, you can learn from Free Code Camp or The Odin Project while building the assigned projects.
If you prefer to blaze your own path, first decide on how frequently you want to build a site. Keep in mind that a greater frequency of building will result in more variety but less depth and vice versa.
Here is a great example of building a portfolio by creating weekly sites: How I finally learned to build stuff with Rails.
And here is inspiration in the form of learning by building daily sites: How to Get Inspired about Web Development.
