For earning online you need to make a blog website or free web.Then start blogging and fill it minimum twenty page.There is motetomize option in blog settings.Adsense is a google affliate network.your blog should be monetomized with Adsense.
You need to open a adsense account.
then you will get a adsense code.you should copy and paste the code on your website.Then you will get advities and clik them.Then per clik per cent.start earning.