For most people in the U.S., coinbase 😎 would be the easiest option to buy Ethereum, Bitcoin, or Litecoin (it doesn't support any others yet). After verifying your account, you can add a number of payment methods including credit or debit cards, U.S. bank accounts, or even wire transfers of funds. Other options for exchanges that will take U.S. dollars for coins are Kraken, and Gemini in the U.S. Typically you will need to verify your account with a driver's license and add other details to expand your buy limits. Since cryptocurrencies are "hard currencies," the exchanges don't want to risk getting ripped off, since you can't reverse a cryptocurrency transaction once it's done...