There are multiple ways to obtain Arbitrum One ETH (a.k.a. AETH or Arb ETH), but here are the best ways depending on your starting point.
In this case, all you need to do is bridge ETH on Ethereum's network to ETH on Arbitrum's network. To do this, navigate to the following site then follow the steps below: https://bridge.arbitrum.io/?l2ChainId=42161.
Make sure your wallet's network is connected to Ethereum Mainnet when using the ETH to AETH bridge.