Get involved in an activity that you enjoy.
If you enjoy hiking join a hiking or a trail maintenance club.
If you enjoy biking, join a group that rides. The local bike shops will have lists of riding groups and when, where they ride.
Take a non-credit class. For example take a class in investing or financial planning or a class on local history. Take a golf or yoga class.
Volunteer. Help plan a fundraiser for the symphony, ballet, or museum. Help with free or low cost medical care for the working poor. Help build a Habitat for Humanity house.
The goal is to increase the number of people you meet while doing something you enjoy. Having a full, active life will make you a more interesting person which will make you more attractive to the opposite sex.