There are to ways to earn Ethereum. First is to join bounty campaign which pays u in ethereum. The 2nd is to join Airdrop &  bounty campaign. From these, u got some valuable tokens, in exchange you can get ethereum. But for that u have little amount of ethereum in your wallet.
I got 150 $ from all Airdrop, but I cannot exchange them because I don't have ethereum.Then I join a bounty which paid me in ethereum.