How do you build a resume to get into extracurricular activities?

Answered

Reading your original question and your response about what ECs you've done makes it seem like you're selling yourself short. You listed three ECs which are related to the programs you are applying for and you've already won an essay contest - that's a good spot to be in! Don't feel like you need to have significant success in an EC just to put it on your resume. You can still learn skills, gain experience, and be an impressive candidate from participating in the ECs. You just need to figure out how to phrase it for the resume.

I took a peek at the programs you mentioned (except for the Senate Page Program, requirements vary for that by senator) and I honestly think you're in a fine spot to apply. National Battlefield Trust Youth Leadership only requires you answer a 500 word essay, get a letter of recommendation, and sign a consent form. Economics for Leaders was similar in that you need a personal statement, LOR, and your high school transcript. Clark Scholars was the most intense but even that only asks for short essays, your transcript, LOR, standardized test scores (maybe that's waived because of covid?), and a list of top-5 volunteer activities. Based on all this I would focus on writing the strongest essays you can for these programs. Considering you won an essay contest I have no doubt you're a capable writer who can handle that.

Building an impressive resume is hard, especially when it feels like you need to have an already impressive resume in order to build your resume. However, you're on the right track. You've shown interest in ECs related to the programs you want so if you're enjoying them you should continue! Gaining leadership positions for those ECs will be helpful. Look for volunteer opportunities and see if you can commit to one or two for a few hours a week or month. If you do volunteer, make sure you stick with it. It's going to look better if you are a consistent and reliable volunteer and not someone who volunteered 10 hours and stopped. If you like writing/are a strong writer considering writing articles on topics that interest you and posting them on a website you make. Even if it doesn't get hits that's ok, you're demonstrating your passion, showing off your writing skills, and people can get a sense of who you are.

Last note, I didn't talk about internships at universities because they are few and far between. Unless your high school has a close relationship with a college or you know a professor or researcher at one it's basically impossible for a high school student to get an internship. Personally I think the time/effort it would take to apply (if you can even find any) is better spent elsewhere.

Happy to clarify anything or answer follow-up questions! Especially if I looked into the wrong programs.