If you are selling goods and services and simply need to know how to accept payments from users on your site, see 'how do I charge money on my web site?' If you are giving away useful information or entertainment, the only practical business model is online advertising.
If you are doing neither -- if you have created a web site with no particular purpose other than a vague wish to make money -- you need to go back to the drawing board, decide what your site is about, and start meeting that need. As with any type of business, wishful thinking doesn't cut it. If you don't have a popular site already, you're not ready to start earning money from it.
The Online Advertising Business
Unfortunately, negotiating advertising details is difficult for small sites. Fortunately, there are companies that serve as advertising brokers, placing ads on your site automatically in exchange for a significant share of revenue. Steer clear of sites that place "popup," "popunder" and other types of obnoxious advertising: these ads send negative messages about your site and are often filtered out anyway by newer web browsers.
Due to such negative experiences, today's web users are much more likely to click on plain-text ads than on any type of advertising. While graphical "banner ads" that do not pop up separate windows are still respectable, most advertisers are not interested in purchasing them due to the fact that users ignore them and browsers automatically filter them. Ad Broker Services
At the time of this writing, the best of the text link advertising brokers is clearly Google AdSense. The ads are typically tasteful, as relevant as Google's search engine results, and very straightforward to install on your pages without any special server-side scripting. The appearance and layout of the ads can be adjusted to meet your needs. Another ad broker option is the MarketBanker service.
Direct Deals With Advertisers
If your site is popular enough, you may begin to receive offers from advertisers wishing to directly purchase advertising links. Be careful! Some advertisers are seeking to confuse search engines by purchasing irrelevant links on popular sites that can be easily confused with real content, which can have negative consequences for your site's search engine rankings when the search engine companies notice -- and of course it hurts everyone when they try to search for something useful.
Other advertisers are offering relevant services of interest to your users, but will make their interest in your search engine rank very clear when negotiating with you. We try to follow these golden rules, in descending order of importance, and we recommend that you do the same: 1. Maintain the positive user experience that made your site popular in the first place.
2. Check out the content the link leads to. Would your customers find it interesting? Does the site actually have content or is it obviously a useless "placeholder" site that the owner hopes to sell based only on its search engine rank? Check back occasionally to make sure a "bait and switch" has not been pulled.
3. Take care to distinguish advertising from the true content of your site.
4. Avoid companies that ask for a long list of "generic" one-word links with common words like "food," "sports" and "travel" unless those subjects are actually relevant to your content. Even then, see rule #2.
5. If the service being offered is genuinely interesting and relevant for your users, the advertiser's motive for buying the ad is not as relevant to your decision.
6. If an ad is "almost" relevant, work with the advertiser to help them get the wording right. That ad for speed dating may be perfectly appropriate for an audience of techies if you find the right way to make it appealing!
7. An ad that relates to your subject matter is best. An ad that relates to other interests you know your visitors share can also be appropriate. For instance, my visitors travel frequently to technical conferences. Hotel ads are a good fit for my site, but cruise line ads would not be.
8. If you are also selling space to an advertising broker like Google AdSense, be sure to follow their terms and conditions carefully and avoid damaging a more important relationship for a one-time direct advertising deal.
9. Insist on payment in advance by wire transfer, or wait for their check to clear before running an ad. If the offer sounds too good to be true, it probably is.
See also Philip Greenspun's "So You Want To Join the World's Grubbiest Club: Internet Entrepreneurs" for a rather successful attempt to answer the more general question, "what should my site be about in order to make money?"
Good luck!