The United States Secret Service do two major things, both of which do not need, or "hire" spies. The USSS are in charge of providing physical protection to the President of the United States, along with his
family and of course his home (the White House). They also provide physical protection to visiting foreign dignitaries. Also protect facilities and major events that the President, his family, or visiting foreign dignitaries may be attending. Basically are very powerful bodyguards. The USSS has many different job opportunities ranging from Special Agents (men and women you see in suits) and Uniformed Police. They have their owned Police Department like the one you see everyday in cities and towns throughout the United States
Secondly, while most people don't know. They also work in investigation counterfeit money (fake money made by people). They are the main agency in charge of this and work like the FBI to investigate and take down the operation during the raid (when they arrest everyone on scene of the crime).
I suggest you look into it and see if it is something you're interested. Unfortunately you cannot be a spy.
They do have undercover agents, either protecting the president by blending in the crowds, or during investigations in counterfeit money making operations.
If you choose to do so then be prepared for a LONG, busy, stressful process. You will need to fly to Washington DC if you even get an interview. You might have to live their depending on what you want to do. You will have a thorough background check, and everything you do during that process will be observed and judged.
Make sure it is something you want to do and make sure you know what it is they do.
You don't have to worry about military background although it does help. They also want young men and women who are physically fit and capable for the important task of protecting the President so also know that the only want the best of the best.