When you make a transaction on Ethereum, you need to pay a network fee (gas fee) to the Ethereum miners. Just as your car can’t run without fuel, your transactions can’t be executed without gas. What is the network fee? The network fee (gas fee)= gas price * gas amount. For more information, you can learn from Ethereum Foundation or a third party service, such as ETH Gas Station.
In short, if you don’t have enough ETH, you can not use the Ethereum network, therefore you can not make any transaction.
You can buy Ether on crypto exchanges or using our browser.