Clients will pay to have a site on the World Wide Web, earning money for the large Internet service providers who rent out Web space.
For smaller operators there is a market for buying Web space and 'subletting' it at a profit. For example, you could buy Web space for £x per megabyte and rent it out to a company at double the price.
First find a reliable provider of Web space.
There are many web hosts who make it very easy for you to rent out web space.
Next you need to promote your business to potential customers.
You can set up a Web site to do this, highlighting the benefits of your service (quality of service, speed of access, reliability or price, for example) and giving testimonials from satisfied customers.
Also, target non-Web using companies by direct mail and other means, explaining the benefits of being online.
The principle reason for why this idea (and a few other ideas here) is so successful can be best understood if you can answer the following question.
Who made the most money out of the great gold rush in America. The prospectors who toiled in rivers of mud day and night, or the store owners who sold the prospectors the essential equipment like shovels and buckets?