[Sangetsu]

There are "free" lessons available in most places, though they aren't really "free". You need to pay an entrance fee of 1500 yen or so, and purchase a textbook, so even though there is a cost, it isn't very much. The bad thing is that these free schools tend to be crowded, with few teachers, so you don't get much individual instruction, but still, they are better than nothing.

Larger schools which charge a fee are more intensive, and offer more one-on-one time with instructors, but these tend to be expensive, and they usually want you to pay your tuition up front, rather than on a week-to-week or month-to-month basis. You will learn more Japanese, and more quickly, but at a cost.

I think Metropolis magazine ("Tokyo's Largest English Language Magazine") has a list of Japanese language schools with contact information. You should be able to find it on the web.