So I just read down to the bottom waiting to see the answer - common interests - but didn't.
It's that easy. Find something you love doing, and you'll inevitably find and meet like-minded people. You'll naturally gravitate to one or more in that community and the seeds for friendship awesomeness are yours to nourish and grow.
Personal experience has yielded 20+ year friendships by skydiving, scuba diving, radio control, and martial arts.