Hello and welcome!
The first thing you should do is create an account on
Coinbase, which is one of the easiest, safest ways to buy Bitcoin or Ethereum. They are a legitimate company and are US based.
Here's a referral link -
Coinbase
Through the Coinbase site, you can immediately purchase Ethereum (ETH).
For Dash, you're going to have to buy some Bitcoin (BTC) through Coinbase, send the BTC to an exchange (Poloniex) and convert your BTC to Dash. And then I would recommend moving your Dash off the exchange to a wallet you trust. For Dash, I use and would recommend the Exodus wallet.
Let me know if you have any questions. Happy to assist!