Advertising would be the easiest option.
You could sign up with CPM/CPC ad networks such as Google AdSense, Casale Media, ValueClick Media, Burst Media, or Tribal Fusion, place banners/leaderboards/skyscrapers on your site. You would then be paid either for each time someone views your site (CPM) or for each click on an ad (CPC). The ad networks find the advertisers for you, take payment, rotate/serve/track the ads and take a 20-50% cut of your earnings for doing so.
Another option would be to sign up for affiliate (CPA) networks, find the advertisers and deals you think would appeal toyour users and then promote those deals across your site. You would then be paid for each time a user bought something through your affiliate link or perform some action on an advertiser's site such as completing a survey.
Or you could sell ads directly to advertisers and serve/track the ads yourself using something like OpenX, Google DFP or OIO Publisher. As a basic rule I tend to set direct prices as double what I would otherwise earn through an ad network (so double would be what the advertiser would have paid the ad network before their 50% cut), and then vary the prices by supply/demand. I wrote a short guide on selling ads directly on AdBalance.
You could also sell links on your site but do note that Google frowns on this and some sites have lost their PR from doing so as it can skew search results.
Other ways to earn from your site could be to sell services you could offer, but as to what those services could be is up to you.