Many websites use standard advertisements sizes, so keep this in mind when you are designing. Here are the sizes that I see the most.

• 728x90 pixels-The big banners that you see on the top of pages
• 468x60 pixels-The small banners usually above and below videos (this size is at the bottom of this page)
• 120x600 pixels-Skinny long Skyscraper (this size is at the right side of this page)
• 125x125 pixels-Square Button (Networking ad below and left)
• 88x31 pixels-Little Button usually found at the bottom of pages (ad below and to the right)

Now you need to establish accounts with major advertisers on the web.

• Adsense by Google--This is the best ad provider I have found. I researched several others for this article, but I recommend this one for sure. It is and easy to use, and very powerful tool
• I tried to find other providers for this article, but I couldn't. You may have better luck than me.