You got some great advice here. What I might offer in addition is to really consider what you want to do within those organizations. They are both large with a wide variety of opportunities to do different jobs. I know people who were or are in the Air Force and each had a different career path: pilot, mechanic, doctor, military police, and hospital administrator.
I would recommend thinking about how you want to be of service in these organizations. What skills and talents do you already possess that you would like to use in your daily work? I have a skill and a passion for writing. I found a job that requires that, because doing what I like to do and what I do well helps me get through the times when work is hard. What is your skill? I think that both would benefit from knowing more than one language as well as physical fitness. You should also consider how much you want to travel and potentially be away from home. It feels like you could do both - begin as a Marine and then move to the Secret Service. Anything that you want to do and do well begins with really knowing yourself. What will you absolutely not do? I can't cut into a human being. I just know that I can't. And my motion sickness means flying would be difficult. But I like to interact with people, train people, or write. That would allow be to be a communications person, an instructor, or potentially work on the social media platform for the Air Force. (The United States Thunderbirds have great social media content by the way.)
Best of luck,
1. College (some sort of homeland security degree or equivalent)
1a. Talk to a recruiter, try to get into a program that commissions you as an officer into the Marines after graduation, probably involving ROTC in college
2. Commission as an officer in the Marine Corps.
3. Apply for the Secret Service.
Keep in mind, the Secret Service selection has a very high attrition rate. Most of your career decisions before selection should be driving you to becoming a better candidate. Great physical condition, experience with criminal investigation, and military experience will help tremendously.
Jake recommends the following next steps: