Qualifications
If you care a whole lot about how much money you want to make, then what you bring to the table is really going to matter. Like any other federal employee, a customs and immigration inspector uses the GS Federal Pay Scale (source). The amount of schoolin' you decide to do will directly affect how much money you'll make.
To get on one of the lowest-ranking pay scales, a GS-5, you're going to need a bachelor's degree. We suggest majoring in something like criminal justice or law enforcement.
However, if you really, really can't stand the idea of going to college (or are really, really into saving money), you still have options. To qualify to be a low-ranking agent without a bachelor's degree, you're going to need at least three years of general experience. But you wouldn't earn nearly as much money as you would if you'd been a star student in your undergraduate career and had qualified for a Superior Academic Achievement—that path lands you a one-way ticket to GS-7.
In order to qualify for Superior Academic Achievement, you have to get a 3.0 or higher in your undergraduate courses (source). Not interested in being a star student? Another way to qualify for the GS-7 level is to go to graduate school for one full year. If you stay in graduate school for two years, you will qualify for a GS-9 level. You get the point: It's your brain, not brawn, that'll get you the sweet paycheck.
Almost equally important as receiving your degree is having some sort of prior law enforcement work. Although it's not absolutely necessary, you will be considered a highly qualified candidate if you come to the table with a few years of law enforcement experience.
Alongside being educated and/or experienced, you'll need to be a U.S. citizen, be at least twenty-one years old at the time of application, have a valid driver's license, and have no prior felonies (source).
After you apply and are officially selected for the job, you'll have to complete twenty-two weeks of basic training. Without completing the training, you ain't getting the job.
Finally, to be qualified for this position, you have to be comfortable with what it'll ask you to do. Customs and immigration inspectors have no say in where or when they're re-stationed; they're required to carry firearms; and their job is to uphold and enforce laws, some of which are very controversial and are being scrutinized by a growing number of Americans. Think long and hard about whether or not you'll be okay with all of that before you fill out that application—you'll be happy that you did.