How to make friends in Japan

Go socializing with colleagues/co-workers/fellow students/whatever. During 'working hours' people are doing their job or studying and so although of course they are polite, they are also busy. Most closer relationships are formed outside of the office so that's what you need to do.

Don't be offended by some of the personal questions you will get asked - like your age! These are cultural differences and these questions are considered ok in Japan. If you don't want to answer then don't get mad, just explain that you are little uncomfortable answering that question right now as it is not usual in your culture. Make sure to follow up with another question or topic starter so that your dialogue can continue.

Just smile to people and you will make friends. Simple!

It's not seem so easy to be friend with japanese people, but just talk with them, go out, and I'm really quite sure that you can have friends!

I do agree with Naval on the fact that smiling is one of the best (and universal) way to make enjoyable encounters and spend a nice time in Japan or any other country.

But for making friends, I would definitely recommend to get involved in activities with Japanese people. If you are part of a "circle" then anything become possible and opportunities to meet new faces will just increase with time. From my own experience, I would say that activities such as sport (in a club with a team) or some voluntary work are winning options. People doing voluntary work are really open-minded and the language should not be a problem. If you are dynamic and try to be helpful, you will definitely make some good friends.