One of the early challenges for gaijins in Tokyo is to find a sim card for your phone. Many mobile service providers require your phone to have a Japanese compliance symbol displayed on the phone itself or within the operating system of your phone. Unfortunately, majority of the phones from overseas do not have this symbol – most likely only phones bought in Japan have the compliance symbol. The irony of it when the government wants to promote tourism. However, there are resellers that would happily do business with you.
If you are just visiting Tokyo for holidays, you can easily get a tourist sim card from resellers at the airport or in popular tourist spots like Roppongi/Shibuya.
If you are staying for at least 6 months, you can sign up for a postpaid sim at one of resellers.
I got mine from Yamada Mobile in Roppongi, using the uMobile service (NTT Docomo network)