Answer:
By placing ads / requesting donations. Or, if you happen to be a cyber criminal, you can place an exploit pack which infects your visitors with malware. ;-)
Another way is by shielding content / functionality and requiring (paid) sign-up. Or sell information about your visitors to 3rd parties (this can sometimes be illegal).
Last thing I can think of is dropping a BitCoin generator on your web site. That's about it.
Of course, if you want to sell things TROUGH your web site rather than earning money with the site itself, there are plenty of other options.