placing ads from paid advertisers on a site

I’m helping a friend of mine with her WordPress blog/site. She has quite a few followers and is thinking about placing ads in the sidebar to help generate some money to help pay for the site. I don’t do much of that on any of my other sites because they are in themselves advertising, so I don’t quite know how to advise her to go about this.

If she’s going to be doing ads from specific companies (pay-per-click advertising) what is the best way to do that? Is a plugin needed, or would we just need to take the link code given to her by the advertiser and place it in the sidebar (such as with a widget)?

Do people even advertise that way anymore? Or are more people/companies using services like AdSense?