For better or for worse, most product design portfolios follow a similar format: a short introduction that concisely describes your design career, a more detailed "about" section, contact links, and case studies.
You will need to present a case study when interviewing for a new job, and it is there that you can differentiate yourself from the crowd. If you lack real-world experience, consider an unsolicited redesign of an application you use regularly. Be sure to state that you are unaffiliated with the company but follow the tips below. Quality over quantity: it is better to have one robust case study than several weak ones.
Use your case study to detail your design process. How did you define the problem, what research methods did you use, what iterations did you make, what was your role, with whom did you collaborate, and what were your results? Tell a coherent story from start to finish. Do not be afraid to share setbacks or failures. Instead, demonstrate what you learned and how you overcame them.
Find a proper balance between text and imagery. Nobody wants to read your novella, but you need words to tell a story. Make sure your text is easy to comprehend and grammatically correct. Include polished high-fidelity mockups, of course, but also animations, videos, low-fidelity mockups, and process sketches.
To an audience unfamiliar with your contributions, real-world data is the best way to measure it. State your problem and outcomes similarly: what was the conversion rate before and after your work? How did your solution increase traffic or decrease churn?
Keep your portfolio up to date. It is much easier to incrementally update your portfolio as you complete projects than to dig up all your wireframes and mockups the day before your interview.