Pay-per-click is one of the main ways to monetize a website and is often the first method used by people that want to start something online.  It is not a coincidence that pay-per-click programs are used by starters:

• the idea of earning money each time someone clicks on an ad is an attractive one;
• Adsense, a pay-per-click program from Google, is one of the most popular monetization programs online; and
• most pay-per-click programs are normally relatively relaxed in terms of who they admit into their programs (as opposed to pay-per-impression programs which require a minimum level of traffic, etc.).

That being said, the idea that pay-per-click is an easy way to earn money online is more of an illusion that reality.  Earning money through pay-per-click programs requires the same ingredient as pretty much all other means of monetization; it requires traffic.  Many people starting online focus on the idea that they will earn money through clicks, yet severely underestimate the difficulty of getting decent traffic in the first place.  That in fact is one of the main mistakes made by "noobs."  They focus too early on monetizing their websites, even when their site doesn't yet have traffic.  This is dangerous because it can generate "fraudulent" or accidental clicks and can get publishers banned from programs pretty fast.  If your intention was to make money online using pay-per-click programs and you have been banned from Adsense, you will soon realize that you have been banned from the best pay-per-click program by very far.  At this point in time (early 2008), nothing compares to Adsense.  The Yahoo Publisher Network is a very distant second and is only available to U.S. residents.

• Not reading the terms of service of the pay-per-click programs.  Those terms of service are important and not reading them is inexcusable.  Read them as many times as required until you understand them.
• Don't focus on monetizing your website until you have decent traffic.  Building and promoting a website takes time, so be patient.  Wait until the time is ripe before you place those ad units on your site.
• Not experimenting in terms of where to place the ad units.  Location can make a huge difference in terms of how much you earn.

So you intend to give it a try and make money online using pay-per-click programs...  Here are some suggestions to get you going:

• Research the keywords that have a higher paying rate.  Some keywords are more competitive than others, meaning that advertisers have to bid in order to have their ads shown.  You want to target those keywords so that you can make decent money from each click.  The last thing you want is to make 2 cents or 4 cents per click.  Instead, you want to make 2 dollars or $1.78 per click... you get the idea.
• The basic principles relating to building websites apply here also, meaning that you need to choose a theme that you know well.  This will allow you to build a good site with good content and will help you attract traffic.  Avoid buying "Adsense ready sites."  These won't work.
• Although Adsense from Google is the main pay-per-click program, there are other pay-per-click programs out there.  Look into them also and compare the rates per click.
• As you design your website, save the good places on a web page for the ads.  The best place to put ads is at the top of a page, close to the center.  The basic idea is to put the ads where they will be visible and where, as people read the content on your page, they will automatically fall on the ads.  Experiment in terms of where you should place your ads.  This is the best way to know for sure the optimal location on a page.
• Integrate your ads into your site in order to avoid making your site look spammy.  Also, try to space your ad units so that visitors will always see a set of ads, even when they scroll down.