Bring some basic knowledge of Japanese (and a polite phrase to ask people to speak Japanese), an adventurous spirit, an open mind, and a willingness to try anything you come across.
Everything else you need is probably available in Japan.
Edit: Personal advice is to avoid expats, meet ups, and anything else which insulates you from Japan... Japan is a hell of a long way to go if you just want to pop down to the bar to drink with some buddies!