Oak house provides guesthouses and apartments in Tokyo, they currently have 119 guesthouses with 2046 rooms. They offer many women-only houses so they are great for women. Also, over 50% of their residents are Japanese, so it gives the perfect opportunity to make Japanese friends and learn Japanese.
They frequently organize parties and events too, so there are lots of opportunities for residents to meet up and get to know each other. All rooms are fully furnished and include free internet. They have multi-lingual staff so they can help you in English, Japanese, Chinese, Korean, French and Spanish.
They offer private rooms, shared rooms, compartments (very small/low rooms), dormitories and apartments. Prices start from about 32000 yen per month for a dormitory/compartment to anywhere from about 55000-135000 yen for a private apartment.