If you have a site optimized for search engines (SEO) can result not enough to be popular in Internet, the use of advertising can be a good choice to bring more visitiors.
Once decided the budget to invest let see most common online advertising methods you can choose:
this Google tool allows to easily create campaigns and monitor them. The annouces are displayed contextually to Google searches but also in other Google services like Gmail and Youtube or as banners in the sites that uses Google AdSense.
The basic rule of Adwords is that you pay only for received clicks (cost per click – CPC).
if you have a database of e-mail addresses (for example obtained via subscriptions/registrations) you can use the classic e-mail to do advertising. There are professional services that allows to manage e-mail marketing campaigns such as MailChimp, AWeber, GetResponse.
social network advertising
specifically about Facebook. It provides good advertising options like post promotions and create campaigns through Facebook Ads.
advertising in sites/blogs/forums siti/blog/forums
many sites/blogs/forums allows to insert various forms of advertising (link, banner, article) in agreement with the author.