I work primarily in the UX world after working at an ID firm for 3 years. During that transition, I spent a lot of time trying to figure out how I was going to put together an exciting portfolio. However, I found that while specific skills in graphic design are helpful—and it’s always helpful to read up on color theory, basic layouts, typography—most of what I needed to do was in translating the creative thinking ability that I used in the 3D world into a 2d portfolio medium. Asking the ‘why’ before delving into the ‘how’ and the ‘what’ of the portfolio helped.
So that might mean asking yourself what kind of work you want to offer your future clients. In your portfolio, what do you need to communicate and what skillset do you need to showcase for that to come true? Once the ‘why’ is figured out, the ‘how’ and 'what’s are easier to determine.