How long and where are you planning to stay in Japan?
Are you going to be a student or are you going to work.
For yearly contracts with phone companies, try Rakuten mobile, it's offers fairly good prices for a provider.
If you are a student and don't know Japanese, ask your tutor to help you;
https://mobile.rakuten.co.jp/fee/super_hodai/?l-id=top_carousel_pc_big_campaign_0614_super_hodai(You need to have your own credit card)