There are 3 main advertising models on the web:
CMP (cost per impression) an advertiser pays for a number of ad views. Example: AmEx launches a new credit card. They don't care how many people click on the ad, they just want to create public awareness, similar to a billboard on the side of the road. Cost to advertiser is pretty low, ads are usually purchased in large quantities.
CPC (cost per click) an advertiser pays for a click on an ad. Example: Heritage Galleries wants to attract new visitors to their website or promote a specific auction. Cost is higher than CMP, but the advertiser pays only for actual visitors.
CPA (cost per action) an advertiser pays only when a visitor completes a transaction. Example: Amazon pays for a referral only when a customer purchases something. Advertiser usually pays a percentage of the sale amount.
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Running a website is a lot of fun and a great way to learn about internet, programming, marketing etc.
If you are looking at making money on the internet, I mean real money, not $10-$20 per month, then treat it as any other business.
You need a good idea to start, a good business plan and an investment of time and money (or at least one of these)

On the other hand, it's easy and cheap (free in most cases) to start building websites for fun and see where it goes from there.