Most buying is done through media companies who charge a fee to the selling site for all revenue they generate for the site. Before starting down this path, do your homework. Visit a lot of advertising information sites and read everything. When you sign a contract for advertising space, make sure you know what you are getting. Impressions means that your ad will be displayed to a certain number of viewers.
Traffic is more expensive than impressions.
My suggestion to new advertisers on the web is to make an alliance with a knowledgeable advertising agency specializing in internet advertising. The fee you pay them should come back to you many times over. They will know what works, what space costs and how to get the best deals. But make sure that it is an agency that doesn't have its own ad inventory to sell. You want an independent agency to negotiate with the selling media company on your behalf.