Are you asking how to get other people to run ads on your website so you can earn advertising revenue, or are you asking how you can advertise your website in other places?
If you want to advertise in other places, it's easy. Simply join an advertiser network (such as Google's AdWords) or directly contact the websites where you want to place an ad and ask them about their advertising policy. (But keep in mind -- if the website doesn't display any ads right now and there's no link offering any sort of information for advertisers, it's likely they don't accept advertising.)
Assuming that you're asking how to get other people to run ads on your site: you have two options.
One is to join some sort of ad publisher network, such as Google's AdSense. You place some code on your site and the network will automatically place ads on your pages. The downside is that the network keeps a portion of the ad revenue, but it's a lot less work for you, since you don't have to contact or recruit any advertisers yourself.
The second option is to recruit other site owners to place ads directly on your site. To do this, you must develop a price sheet showing your advertising rates. You will also need to be able to demonstrate to these other site owners that your site gets enough traffic from their desired target audience to make placing an ad on your site worth their while. If you cannot demonstrate significant website traffic, you're unlikely to be successful in selling much advertising space. You'll have to work a lot harder, but you receive 100% of the ad revenue.
If your site is relatively new, or if you don't have a ton of traffic, or if you don't want to go through the extra work of recruiting advertisers yourself, signing up with an ad publisher network is your better alternative. If you have sufficient website traffic and are motivated to do the extra work, soliciting ads directly from advertisers will probably result in more advertising income for you.
Hope this helps!