about cell phones

I'll be going to tokyo on next september and i'll be there approximately one year (until july).

I'd like to get a cell phone over there to be in contact with friends and everything.

I've seen there are various plans available and even prepaid phones.
Is there some option you recommend the most knowing that i'll be there for one year and that i'm a student?? (for example, i know au offers 50% discount to students).

And other question: do you think i could buy a 2 year old japanese phone through ebay and then somehow reactivate it in japan? For example, that i buy the sony ericsson w42s and reactivate it with AU when i get to japan?

Don't buy over Ebay a cellphone. Just go the place in Japan that you are visiting and buy a keitai there. I buyed one some months ago in Nagoya and now i'm in Belgium and now everyone looks like this to me (What kind of cellphone does he have? its full of chinese letters)

so is there any places in tokyo where i can buy a used cell phone (not unlocked or anything) and reactivate it with au, docomo or softbank?? is this possible? or is it strictly necessary to buy a new one?