Adsense is the best. It is easy actually. Depending on how you want to start with.
I started simply by Blogger.com. Post a couple of blogs a day (a per day would be fine). After about 3 days, apply for adsense and you will have a very high chance to get approved.
Once you get yourself approved, then you can just link it to all your accounts like HubPages and even your own blogs.
Hope this helps!