There are a lot of options to get a SIM card in Japan. Some of the major ones include Docomo, Softbank, YMobile, and LineMo (online SIM). To sign the letter of consent, you will need to be at least 18 years of age. Most importantly, usually, they need you to have a credit card as the contract payment will be paid by credit card. It is better for you to have a Japanese one as some of the international credit cards do not work. If you do not have one, do not worry! Some carriers provide options for you to pay in convenience stores every month.
https://www.softbank.jp/
https://mobile.line.me/
https://www.docomo.ne.jp/