I think you've gotten some great tips above about using your existing resources/network at school to seek out opportunities. I am delighted to see your enthusiasm and head start in the field. Truthfully, I'm not sure how often companies may hire high school students for internships (it is an incredibly good idea, however, to do this as a junior/senior in college as you'll have your foot in the door and interns may end up getting hired). You may want to do some searches on your own by simply going on websites of companies you admire or have your eye on (e..g, Amazon, Google, etc) and see the types of job descriptions and interns available for the summer. You may also want to go on job search websites such as GlassDoor and plug in "Marketing Intern" and "Atlanta" into the prompt fields and see what type of positions turn up and what sounds interesting to you. You may want to reach out to those companies by email (go to their sites, find out who their talent management and hiring departments are) and simply ask if there are any opportunities for someone to intern at your level. Even if they aren't hiring, they may know someone else who potentially would be. I'd say quite a bit of networking plus research will be your formula.
If you can't find a marketing-specific opportunity at this stage, I'd still take full advantage of your intention by checking out the type of requirements and job descriptions that are necessary for these internships. Then find ways -- either at school or in your community -- to somehow get experience. For example, are there specific computer programs that everyone is asking familiarity with? Are you familiar with them?
Don't forget - you can practice marketing YOURSELF too -- create a resume (this is your own marketing tool) and create a little marketing plan for yourself -- what will the next 3 years look like? What are your goals and how will you get there? Who do you need to get in touch with? Give yourself some specific steps to follow. Figure out what your key strengths and unique personality aspects.
Congratulations on all your achievements thus far and your future goals.