I am reserved naturally but have been working on this for about 15 years. For me it is easier to be social when I have my kids with me. People seam more friendly when they can look at your kids rather then you, the mom. However I think I have become a good conversationalist with out my kids too. Meeting people at kid activities is easy. But you say your kids are all in school, so I would pick a few things that interest you: art classes, the gym, volunteering at the school or library, knitting groups anything that you would like to do. That way the people you will be around already have something in common with you. Most people like to talk about themselves. For that reason, it is so easy to start conversations with a question. Anything really, like how are you today? How many kids do you have? What is your name? If you see them at a certain place often you can comment on that too. "Hello again, I see you here so often, what is your name?" Or "I need your name so I can say hi" etc... If they are good conversationalist too, they will return the questions and Wa-la you have a friend. After a few meetings and conversations you can exchange phone numbers and meet for other things or perhaps get your families together. Another great place to meet people is a church. If you have a belief in God, then you will already have something in common with all the people there.
I have found the best thing you can do for meeting people is to make yourself approachable. Smile at people you pass by and look and be friendly! One of the most comforting things is when someone talks to you and they look into your eyes, and seem genuinely interested in what you are saying. Do this to others and you will see that people will want to be around you. It is amazing how many more people will talk to you if you make eye contact, smile and say hi.
You can do this!