The US requires all passengers, including those in transit, to go through customs - probably a security thing or maybe they haven't caught on to the idea of a transit lounge?
Consequently you'll need to check the visa requirements relevant to you. Most western nation citizens can get a transit visa on arrival, but it depends on where exactly you're from.
Regardless of visa requirements ALL passengers need to complete an ESTA it's a security clearance thing. You must do this at least 72 hours before you depart. You can do it online and it costs $14 (in my Aussie currency). If you don't get one you won't be allowed onboard at your point of departure.
And after that why not check out our other travel tips for the UDSA, such as How much to tip?
Phil at the safety hub about 7 years ago