Portfolios are a great way to create a collection of your work. Strong portfolios can land you a job or even get you into college.
Tips for portfolios
Use lots of images. It will be much easier for an outside person to know what you're talking about if you use lots of pictures. They help clarify what you're talking about and show the extent of your skills. Images can also make your portfolio more interesting and keep the reader engaged.
Tell a story. It's very common to run into problems in engineering. If you're able to tell a story about how you overcame an obstacle in order to make great work, possible employers or colleges will be able to see how well you work through problems. This is also another strategy to keep the reader engaged.
Cater your portfolio to your intended audience. Thinking about who you're making this for will help you include the most relevant information for your situation. For example, if you want to send your portfolio to colleges, you may want to show a strong desire to learn new skills. If you want to use your portfolio to land a job, you may want to showcase a strong mechanical aptitude and lots of experience.