As someone who has dabbled with my own websites, I would say there are basically 2 ways to generate revenue from your site:
1) Affiliate marketing - you place ads for a company on your site, a visitor clicks on it and then buys a product off advertisers website. You will get a percentage of the sale price, generally no more than 5%, but can be as much as 30% or 40%. Or sometimes you may recieve a fixed amount.
Personally I was very lucky to get a click and never got any commision at all this way!
2) Google Adsense - you place Google generated ads on your site, somebody clicks on them and you recieve a set amount based on whatever the advertiser is willing to pay per click/per page view.
I managed to make enough this way to make the websites pay for themselves and make a very small profit.
You will never make a huge amount from online adverts unless you come up with a very popular national/international website!