Hi Ashleyn. I'm a vet , also had a family member in the Secret Service and another one is a cop. Here are some insights.
The Secret Service virtually requires a college degree. So if that is a goal for you, you'll need to graduate from college. (show you how in a minute). And it is a high stress job.
Often SWAT teams are a specialty in local police departments. To get there you first need to be a cop, and for that you often can apply if you have a high school degree and are 21. If that is appealing then literally go to your local police department and see if there is someone off duty you can talk to about a career in law enforcement and what it takes to join the SWAT team.
For the military....it might be a good option. It is for many folks. Here's why I suggest that. You can enlist when you reach age 18 AND....if you stay in a while (six years I think) there are numerous federal and state government programs that will help pay for the cost of college. So, that's how you might qualify for the Secret Service if it is still an interest for you. Regardless, being a veteran opens educational and career opportunities hard to create otherwise.
Hope that helps. Good luck my friend.