A Bachelor's degree is required for federal special agent positions (very few exceptions). Any responsible employment is acceptable, but law enforcement may be beneficial. Any major is acceptable, but Criminal Justice, Sociology, or Psychology may stand you in better stead. Those meeting the minimum requirements may not compare well to the best qualified applicants, and there are always more applicants than positions. The FBI likes lawyers and accountants, but they hire from various backgrounds, with at least 3 years of substantive employment ((http://www.fbijobs.gov/).
Other federal agency job announcements should be available at https://my.usajobs.gov/login.aspx.
I recommend that you major in psych and minor in CJ (even though I majored in CJ). GPA, testing, physical fitness, work experience, possibly a polygraph, ability to communicate orally and in writing, and graduate degrees are what determine who gets hired. And, I repeat, there are always more applicants than there are positions.
There are numerous special agent positions (e.g., DEA, ICE, NCIS), in what was and I assume still is the 1811 job series (1810 are unarmed investigators without arrest authority). There are also law enforcement related positions in the Dept of Homeland Security and other agencies, such as inspector positions or Border Patrol agents, that could be open to those with 2 years of college or less. And, there are federal police officers within federal agencies (e.g., Dept of Veterans Affairs, Treasury, Federal Protective Service).
Air Marshals work for the Dept of Homeland Sec, a very boring job (one who did the work likened it to the movie Groundhog Day). Alcohol Tobacco & Firearms (bombing, arson, felons with weapons) is in the Dept of Justice. You might also consider the U.S. Marshals Service. They have two divisions, one is responsible for fugitive investigations, and has the same grade structure as other major federal agencies. Most federal agencies have Inspector General special agents with varying authorities. The State Dept also has special agents, as does IRS (they do tax evasion and money laundering, which is not as boring as it sounds). State Dept agents ride desks overseas, and do protective details (other than heads of state), visa and passport violations in the US. There are even Fish & Wildlife special agents.
I attended Secret Service protective detail training and participated in a protection detail for the Commissioner of Customs. Protection details require meticulous planning and execution for events that almost never happen, and they are boring. USSS protection details require frequent travel, making for poor family life. The investigative jurisdiction of the USSS is not all that intriguing (counterfeiting, computer crimes, financial crimes).The FBI offers the opportunity to perform the widest variety of investigations, with the possibility of specializing in an area of interest, or working in an Embassy as a Legal Attaché. I was an acting Customs Attaché in Panama for about a month, and it is interesting if you like that sort of thing.
My general advice is to apply at as many places as possible, and accept the first offer. Then, if that’s not where you want to be, keep applying. It is not unusual to find former agents from some other agency working at another agency, and it is easier to get a job if you have a job.