It’s one of the oldest and most proven ways to make money – buy low, sell high. The buy low part comes from searching garage sales, estate sales, and even thrift stores to find items that are in good condition (“gently used”) but selling well below what they would if they were brand-new. In this way, you might be able to acquire an item for $5, and later sell it for $50.
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.