First create a website. Then apply to Google Adsense https://www.google.com/adsense/ You earn everytime a user clicks on the ads you show.
You can also apply to Yahoo Publisher’s Network, but this program is still in beta and only accepts US publishers. http://publisher.yahoo.com/ Note however, that you cannot put in the same page Google ads and YPN ads as it will be a violation of Google’s terms which specifies that you cannot use competing contextual advertising network on the same page with Adsense (but you can use them on other pages where you don’t show Adsense)
In terms of earning money on Adsense, your mileage varies. One website with the 10,000 uniques a day can earn $50 a month while another may earn $5,000. It is not easy to predict how much you will earn from Adsense. The only way you can learn about how your site will perform with Adsense is through trying it.
The amount you can earn will depend on the
1. Responsiveness of audience to the ads = A travel website that provides information on travel to Spain will attract visitors looking for ways to arrange their travel and spend money on their vacation to Spain. Your site provides the info, but the ads will provide hotels, travel agencies, tourist destinations, car rentals — ads that are likely to get the attention of the users of your site. This is a site that will most likely do well with Adsense. However, if you are a gaming website where the main purpose of the user is to play games on your site, then Adsense will not perform as well.
2. Ad format = some types of ads do better than others depending on your content and layout. In our case, large rectangles in the middle of the content is the best, while leaderboards do not generate as much as income. Skys are the worst for us. Experiment and measure the results via channels and see which formats work best for you.
3. Ad placement – check Google’s heat map as they have tested where the best placements are https://www.google.com/support/adsense/b…
4. Ad colors – sometimes ads blended into the content works wonders, but sometimes ads that contrast your site colors work best
5. Number of ad units on a page = we are allowed maximum of 3 ads + 1 ad links + 1 search box on a page. Maximize the allowed number based on the resulting look of your page (you don’t want an overkill of ads). Users going to your page and reading your content may ignore the banner or rectangle at the top of the page, but may click on the ad at the bottom of the article
6. Smartpricing – the big unknown in Adsense. No one knows how this actually works. But it can affect the pricing of the ads on your site. If the advertiser paid for $0.50/click – but your site is smartpriced – then the cost may be discounted lower (e.g. $0.25). So you may try to develop a site based on high paying keywords but if smartpricing gets to you, then you may not get as much per click as what you are expecting from your keywords.
Here is Google’s explanation of smart pricing https://adwords.google.com/support/bin/a…
Google’s smart pricing feature automatically adjusts the cost of a keyword-targeted content click based on its effectiveness compared to a search click. So if our data shows that a click from a content page is less likely to turn into actionable business results — such as online sales, registrations, phone calls, or newsletter signups — we reduce the price you pay for that click.
Experiment with the factors above (except smartpricing, which you can’t control), and see which combination works best. Remember though that not all sites do well with Adsense – even if you get gazillions of traffic but your visitors are not interested in looking for ways to spend their money, they won’t be interested in your ads and won’t click.

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