First you need to set your site properly, then you could start giving ads, google adsense is not a bad idea, you can also go for that only if you have
traffic > 1000
Well there are many others but by far adsense is the best for a beginner due to a number of reasons. I mean it is reliable (though a little strict), and it pays well.
To generate revenue from site you need to have good traffic sites. Once get good traffic and high pr start selling adspace like selling text link to individuals or by placing ad unit through adsense, YPN.
If you get good amount of traffic and your site is focused on a niche you can sell advertisements directly to interested companies, beside this you can also sell links once your site has received good PR.