There are plenty of 100 ¥ shops where you'll find enough to survive. You can eat Cup Ramen for less than 200 ¥ (but not so healthy) or Gyuudon for less than 300¥ and Tendon for less than 500 ¥ in their respective shops (look for Yoshinoya or Matsuya gyuudon or Tenya for tendon). Combini (24h convenience store) such as Family Mart, AM PM, Sun-kus or 7-Eleven have cheap sandwiches and other ready-to-eat dishes (ask the staff to warn it up for you). An even better solution is to buy o-bento (lunch box). They usually cost between 400 and 600¥ and thre are dozens of varieties, such as subuta (sweet&sour pork), sukiyaki, katsudon, oyakodon, maku no uchi (assorted) bento, kaki-fry, mabou-toufu... Lots of small bento shops open only for lunch time (12.00 to 1.00 pm), so any other time look for chains like Hokka-Hokka Bento or the 24h Origin Bento.
To give you an idea of prices in restaurant, you can get a good ramen or pasta for about 500-900 ¥. A 12 sushi set in a supermarket will cost between 500 and 1000 ¥, but you can have them half price after 8.00 pm in many places. If you have a place to cook, you can buy about 10 gyozas for 250 ¥ in a supermarket.
Buying books in English, French, German...
The cheapest way to buy new books is through Amazon Japan, which is available both in Japanese and in English.
You can also buy used books or trade your books on our market square. If you still prefer to go to a bookshop, the best English, French and German selection can be found at Maruzen, Kinokuniya, Book First and Sanseido.
Buying a mobile phone
There are presently three mobile phone providers in Japan: NTT Docomo, KDDI AU and Vodaphone. It is good to know that you can only send cheap short-messages to people who use the same provider as you. You can also get family discount or get reduced tariff on calls to one chosen person using the same provider. So it will more convenient for you to choose the same as your friends.
Getting an Internet connection
There are many internet companies offering ADSL, among which Yahoo BB, KDDI Dion, NTT and your area's cable TV. It is important to know whether you need to have a hard line (i.e. a fixed phone number) to obtain an Internet connection. Otherwise it will cost you over 30.000 ¥ in installation fee to add one. The Cable TV ADSL is a bit different. It also requires an installation, but not the same as the phone line. It is a bit cheaper and advantageous if you intend to watch cable TV, as it is the same line (but the fees are separate, allowing you to choose only the TV or only the Internet if you wish). Another advantage is that the speed doesn't not decrease with the distance to the company's office, as it does for NTT and those using their lines (Yahoo BB...).
An alternative to home Internet is going to a manga kissa (manga cafe). Most of them have computers with Internet and various softwares or games, which you can use for a flat fare (around 300¥/h), along with reading mangas, magazines or even watch DVD's or cable/satelite TV in some places. Most of them are open 24h and also have confortable reclining chairs convenient for naps or even to spend the whole night instead of having to pay for a hotel when you miss you last train. Some even have showers.
Buying a computer
If you go to big electronic shops such as Sakura-ya, Bic Camera, Yodobashi Camera or Ishimaru Denki, the computers will be primarily famous (and expensive) brands like IBM, Sony, Fujitsu, NEC or Dell. To find cheaper, brandless material, go to Akihabara. It is difficult to recommend a place in particular, as prices are generally comparable, but you will have two options if you are looking for a cheap PC: buy it piece by piece and mount it yourself, or buy it second-hand. Some shops are specialised in second-hand PC's, including separate parts.
Buying electronics & electric appliances
Big electronic shops are great places to see and test what's available. Lot's of them have member's cards which give you big discounts on your future purchases. The normally all have a delivery service, to avoid you carry bulky stuffs home and don't have a car. It is usually included in the prices for such things as refridgerators or washing machines.
The cheapest way, however, is probably again buying on the Internet. Try Rakuten Ichiba (only in Japanese).