You don’t have to buy a full ETH or any other full coin when you invest. You can buy 0.5, 0.1 or even 0.001 of a coin. Don’t invest more than what you can afford to lose.
As of today, in order to buy some ADA, firstly you have to buy some Ethereum (ETH, faster and cheaper transaction fee than Bitcoin) on a platform like Coinbase or Bitpanda, then move these ETH to Binance (cheaper fees 0.1% per trade) or Bittrex (higher fees 0.25% per trade), then convert these ETH to ADA.
Next step in order to safely store your ADA is to send them to your Daedalus wallet on your computer.
Technically speaking, they won’t be stored physically on your computer but on the blockchain, but at least you will be the owner of the private keys (also called “seed”, or “secret words”) to access your coins.
DO NOT store your private keys/ seed/ secret words on anything connected to the internet. Write it down on 2 sheets of paper, one safely hidden at your home, the other at a family member you trust (in case your house burns or is visited by thieves). You are now responsible of your money’s security.
Feel free to ask questions, and don’t forget to triple check your address when you move your coins, and do a test transaction first with a few ADA to build confidence