It's hard to think of a 13 year old building their resume, but that's the case in preparation for high school. The good thing is, most likely you're not starting from scratch! Most students have already gotten involved in sports, lessons, or other interests before they enter high school. If not, that's ok, too! Here are some guidelines in what to do each step of the way to build that resume for high school jobs, internships and the college application.
Continue to do the activities that interest you from before high school - were you heavily involved in music or competitive swimming? If you still love it, continue it! Silly as it might be, activities on the resume only count (for college application purposes) if you've continued them into your high school career.
Try lots of different things from different parts of life. Get involved in school clubs and sports; look forward opportunities of interest within the community; consider a part time job (if legal where you live); look for volunteer opportunities at hospitals and other non-profits. Try a range of activities both inside and outside of school (assuming you have that freedom and ability to do so)
Some popular activities that I've seen students experiment with: tutoring younger students, learning to program, volunteering at a hospital, getting involved in other community based non-profits (food bank, senior centers, etc.), new sports, new instrument, school clubs (or starting one!).
Really let interest and enjoyment lead the way in exploring the different activities. While you don't want to quit after the first meeting, you also don't want to stay beyond your true interest's expiration so that you can dedicate more time to activities that you truly enjoy into the next few years.
Start sifting out the activities that are not worth the time that's put into it; for example, many times sports fall into this category where the love or skill level of the sport won't outweigh the 12+ hour commitment per week a school sport requires
Consider moving into entry level leadership for the activities that you're in. This might look like taking on a treasurer or secretary role at a school club or taking on larger projects at a non-profit or coaching younger athletes of the same sport. Leadership doesn't have to be traditional, there are lots of ways to have big impact without a title (just as there are individual contributors and many industries that are very important such as within the engineering world). This might look like starting or vastly increasing success in a fundraising for a club at school or taking on full responsibility for a sensor project for a new robot in your robotics team.
This is when you might narrow down even further and deepen interests and increase leadership in the activities in which you're involved. This will look different for each person and each activity. For a cellist that might mean taking on first chair, coaching younger students, teaching private lessons to younger musicians, and competing more frequently. For a school club member, that might mean taking on VP or Presidency 11th or 12th grade, starting new projects/ideas for the club (new fundraiser, new ways for the group to be more productive in its goals), or train new members to take on a leadership role you might be leaving in order to take on a bigger role.
Don't waste them! Do something with them.
Consider deepening interests in one of the activity areas you most enjoy. For example, if you love to volunteer at the hospital, consider looking into summer programs that help children that are hospital bound or look into shadowing a doctor. Or, if you are a swimmer, consider taking a life guard course or exploring other water sports.
If applicable, summers can be a time to explore possible future careers. If you're interested in becoming a dentist (as I was growing up!), you can ask a local dentist how you can volunteer in their office and learn more about what their day looks like.
Of course, there are incredible summer programs put together for various activities (science research, robotics, math, writing, music). The general rule of thumb for these programs is, the less it costs, the more its worth on a resume (typically because the more competitive it is to be part of). Typically, these programs will have applications open during the winter of the school year prior and are great practice for future college application essay writing.