Go to ENS manager and search for the .eth domain you like. Eg. talkmarkets.eth
Make sure to load your wallet with ETH. Total costs including gas on average is $70-$90.
After you found your domain, connect your wallet and follow the steps on your screen to buy it.
When the purchase is confirmed, you will be asked if you want to create a reverse record. You wanna do this if you want your .eth domain to show up on etherscan.