Google Adsense
Google Adsense is probably the easiest way to start earning money from your website. You can sign up for an account here (its totally free to do so). Once your account is live you can go into the adsense setup wizard where you can create an advert to display on your website - you can customize the colouring, size and position of the adverts to match your site layout and you can also create multiple adverts each with their own names and tracking facilities - this means you can put different adverts on different pages and then you can track how well each of your pages is performing in terms of advertising revenue.
Once you have created your advert design, you will be given the code, simply place this code into your website - based on where you want the adverts to appear, save and upload the page in question and your site is now ready to serve adds!
The genious of google Adsense is that the code will automatically choose what adverts to display based on your content, so if you run a hosting website, Google will display adds related to this subject. The benefits of this are that the adds will automatically be targeted to your visitors - which means they are more likely to click on them. Every time a visitor clicks on one of the adverts, you are payed a certain amount of money (this figure is greatly dependant upon the industry that your website covers).
In your Google Adsense control panel you can keep track of how many times the adds have been displayed, how many clicks you have had and how much money you have earned per click. Earnings are payed to you by check once a month but the sum total of your earnings has to be above 100 US Dollars before you can make a withdrawal. If you prefer to save your earnings you can put the payment on hold and the money will keep mounting up.