When fueling up at a Japanese gas station (whether self-service or full-service), look for the color-coded system:
レギュラー (Regular): Red nozzle/signs. This is exactly what you need.
ハイオク (High Octane / Premium): Yellow nozzle/signs. (Not needed; using this won’t give you extra performance or benefits).
軽油 (Diesel): Green nozzle/signs. (Do not use this under any circumstances).
If you are using a self-service station (Self / セルフ), just touch the Red button or screen option labeled レギュラー to start your selection.
*** IMPORTANT*** Please double check you chose is correct, before paying. Adding any other types gas will damage the car engine. You will be liable for any damages.
Also, In Japan there are self-service (セルフ) and full service. This is the traditional style where you don’t have to step out of your vehicle. The staff handles everything.