I'm in grade 12 this year and I have very good grades in english. I really like japanese culture, and would love to live there some day. In the newspaper, I saw something for teaching overseas, and it said that a degree is not required. Is it possible to teach english with just a high school diploma?

For a job in a small language/english school, all you will need is a 4 year University degree and to be a native speaker. In such cases, you cannot be picky about the work hours and pay.

For something better, having a teaching degree and TOEFL, also maybe participating in a teachers programm will probably give you more opportunities for better paying jobs.

There are a lot of foreigners who come to teach english in Japan, so a lot of competition. And unfortunatly even some big and famous schools such as NOVA have had troubles with mistreatment of their foreign staff (too many work hours, low pay) and in some cases bankrupsy without paying their staff's salary.
You should be very careful in your choice.

If your interested in the culture, but have never really been in contact with a japanese environment yet, I suggest you take a short (maybe 3 months) vacation to Japan first to see if teaching here is really what you would like.Teaching english is not the only opportunity for foreigners in Japan.

In my 9 years living here in Japan, I have met many foreigners who had many ideals about the country, loving it's culture before coming, but many left with their illusions broken and hating the country in the end.

Living in Japan, as in any foreign country, is not an easy thing. It can get hard mentally sometimes, so I strongly suggest you be well prepared so you can have a good time and not regret your decision in coming.

Good luck!

N.B. A High-school degree will not be enough for you to get a working visa. For the working visa Japan requires a 4 year university degree. If you come over on a student visa, you can probably find a low pay part-time job baby sitting children in english. But you will have to be at school full time to keep your student visa. (this includes coming to study japanese in a japanese language school/university. But may be expensive)
If you are from Canada/Australia/France/Germany?/Korea you could use a "working Holiday Visa" which will give you the opportunity to stay and work in Japan for 1 year (this visa can only be used once and depending on your country the age limit is up until 25 to 30yrs. old)

yes but you need a TOEFL certificate to get a good paying job