Unless things have changed a lot since my day, you'll be asked to pick several choices of location, and you may not get first choice.
In addition to Yokota, you might put Kadena (Okinawa) for your second choice. You'll almost certainly find kendo near either base, and I can guarantee you'll find karate near Kadena.
Wherever you get stationed, ask around about what's available and try to actually visit different schools. That's the best way for someone with little prior experience to get a feel for what "fits" him or her.
Good luck in the AF. There's a bit of my blood, sweat, and tears on the ground at Lackland from 3 decades ago. If you get a chance to visit the Security Police Museum they may even have my old white service cap I donated (we switched to blue berets while I was there).
Yours in Budo,
---Brian---