There are many ways to earn Bitcoins.
The fastest way is to convert some of your regular currency such as dollars or euros into bitcoins. There are may webs that offer a service known as automated exchange that allow you to convert between several currencies available on the market.
Like with any other currency, you can work and get paid in Bitcoins. There are some websites out there that offer job listings that pay in Bitcoin.
Or you can sell stuff you don’t need for Bitcoins, the same way you sell stuff on eBay.