As somebody who became single in his mid-50s I strongly second the recommendation for meetup groups.
They aren't exclusively for single people, but singles are very "over-represented".
Obviously you want to choose the groups with a large number of people of your favorite gender group.
For me (looking for a woman) the hiking and the arts related groups were good.
Even if you don't find dates, you'll likely have a good time and meet new friends.
Meetup groups give you a low-stress way (because they're not primarily dating in general) to interact with many people
Also, in response to a different comment. I think that lots of women in their 50s are actually rather
computer literate. (At least the educated women around the major city I live close to.)
(My personal experience is that I met someone via a meetup group, but I wasn't ready for something
as serious as she was looking for at that time. Shortly after that I met someone on OKC and we're still
together 2 years later.)