Which app are you using to manage your nominations? You don’t have to nominate through Polkadot.js, but there are several advantages including a more granular control and view of what’s happening on chain, and no third party fees.
There are several ways to add an account to polkadot.js depending on what you want to use it for. If you are looking to nominate and the amount is non-trivial, I recommend creating two accounts, a stash a controller. The stash account simply holds the funds and is more secure, offline, “cold storage.” It’s less convenient to use to sign everyday transactions, so for that you use the controller account.
Two options for cold storage are parity signer and a ledger device. One thing to note is you will need a browser like Chrome or Brave to connect ledger to polkadot.js. For a controller account I’d recommend the polkadot.js browser extension. Here is a tutorial showing how to create a parity signer and extension account.
One note is that if you have less than 200 DOT you will probably want to stake through a custodial service like Kraken. More information here.