Fiat On-ramping

Find a website that supports buying of Bitcoin (or Ethereum) with U.S. dollars. I use https://bitstamp.net and they charge 2% when using a debit/credit card (so a $500 deposit will generate an additional $10 fee). Other popular sites are Coinbase (higher fees?) and Bittrex.

Assuming you use Bitstamp, create an account and buy Bitcoin (BTC). They make it pretty easy, but it can take a day or two to get the account created.

After you purchase Bitcoin, you will need to transfer it to another crypto exchange in order to trade it for additional altcoins.