You sign up for google adsense, chitika or just google advertising advert networks, there are plenty of them, but google adsense is probbably the best, esp considering u want those adverts with cars...lol
Anways the good thing abt adsense it it delivers ads that are related to the content on your website.
You can also contact local companies like econet, telecel and so on, and if u have good traffic, u might get them to put adverts on your sites,
As to how to put them on your site, it depends n which platform its running on. If its CMS like wordpress and joomla there is plugins for that. If its hard code, u wld have to leave free space for the ads.