This manual explains where to get Metamask, how to set it to the Sepolia testnet and how to mine some Sepolia Ethereum that can be used to mine/list/sell NFTs on this site (and everywhere else that uses Sepolia Ethereum).
If you don't have metamask, you can download and install it here.
Set up metamask, then change the blockchain to the Sepolia testnet by clicking at the top:
Click the button next to the account name:
Activate the "Show test networks" option, then select the "Sepolia" network.
Now, some SepoliaETH is required to do stuff on the blockchain. You can mine some here.Here's what each page is for: