The big communications giants like Ameritech, Bell, Sprint and others provide communications links to smaller firms that are called National Service Providers and they in turn provide links to smaller firms known as Regional Service Providers and they in turn provide links so that you and I can market our products and services to the world via the Internet. Now the hard part. Many of them are aimed at a specific audience like educators, researchers, civil engineers, etc. Some are industry specific. Many industry groups have their own network.
Some providers claim they reach everyone.
There are some publications available to guide you through the maze. One in particular is The Whole Internet Users Guide and Catalog by Ed Krol. You might also try the Internet World magazine. I'm sure there are others. There also are firms that conduct training classes for those wanting to understand the Internet.