I believe the basic rule in Japan is that if you do not have a residence permit or proper visa, you cannot buy a sim card or prepaid phone. So my first question is: are you a tourist or staying more permanently?
If you are a tourist: go for a rental sim card, or try to convince a Japanese friend/foreign friend with proper visa to get you a prepaid phone.
If you are staying more permanently: bring a Japanese friend to the store, or go to a branch with English speaking staff. In Tokyo for example, the Softbank Omotesando branch is usually where foreign students go to for help in English.
If it's just for a matter of texting and limited phone calls, I would not bother looking further and just go for the Softbank prepaid plan. For about 300 yen you can activate "unlimited text messages", which keeps you phone expenses very low.