Go out to drink to socialize.

Japanese people like to be in groups.
They also take close attention to "Kuki" (air) the atmosphere of the location, the people they are with. You won't be welcomed if you bombard a random group or at an Izakaya.

There are many pubs all around Japan now.
These places are more open, the Japanese people will know before hand that they will not be able to keep their own atmosphere.
I'd recommend to go to these places and approach someone to talk to.

Also there is a "MACH KON" it's a gathering of large groups of people that do a pub craw or Izakaya craw.
People with the same band around their wrist let's everyone know they are comfortable talking to these people even if they are complete strangers.