How can I show colleges I am interested in pursuing law?

I would strongly suggest joining Mock Trial if your school offers it as a club. Otherwise, google search opportunities to watch court hearings and trials - I don't know the specific requirements for that but it could be a possibility to gain more exposure.

One of the best things you can do is take as many social science classes as you can. As @sak_26 said, joining Mock Trial will do wonders, also join things like Student Government(aka Student Council), Congressional Debate, Debate, Speech, and then internships like @SunnySunshine said as well.

One of the main things that is going to indicate to colleges that you want to go into law is advocacy and community involvement. So, if you can, participate in your school's NHS chapter. If you get invitations for membership in other honor societies like NSHSS or Honor Society, make sure you jump at the chance, because that community involvement will look pretty good so long as you are actually involved. There are also non-profit organizations like Luv Michael that you can fundraise for, and if you fundraise $100, you get a certificate reflecting your service hours, at $250, you receive a grad cord, and at $500, you receive a stellar recommendation letter.

Overall, just really focus on your public speaking, your presenting, your involvement in student politics, and your community involvement. This will show colleges that you're passionate about law and things related to it.

For law-related opportunities, you can intern under a political candidate during the election season, this can show an interest in politics, and you can get a really good recommendation letter, too.

I saw a TikTok that this girl had sent emails to a lot of law firms to intern at or shadow! she got a lot of responses, mostly positive and inviting her