Probably two of the most important factors are your appearance and your personality- you need to look professional and well-dressed- the Japanese work environment is still a lot more formal than in English-speaking countries.
You need to be outgoing and well-spoken but still come across as very professional. If you are applying for a kids teaching job it is of course important to seem fun, but not airheaded.
These days a TEFL qualification of some sort is becoming more important, and not just an online on, it needs to have a practical teaching component. Any teaching experience you can put on your resume is great, even if it's just volunteer. Japanese language skills are likely to open more doors. Any travel you can do (not just to Hawaii or Acapulco for example) may be helpful because it shows you can deal with unfamiliar situations and won't leave suddenly due to homesickness.
Learn English grammar, and make sure your written expression in your resume and cover letter is excellent. Any spelling or grammar mistakes are going to mean your application is binned.
If I remember correctly you're still at high school? You have plenty of time anyway. Have a look at dedicated TEFL/TESL sites on the internet, like the Japan forum of Dave's ESL Cafe for example-
Dave's ESL Cafe .
Read through the threads there and you'll find answers to a lot of your questions.