To use Nostr, you will need to have some Bitcoin in your wallet. You can either buy Bitcoin from an exchange and transfer it to your wallet, or you can receive Bitcoin from someone who already has it.
Regarding the transaction fees, there is typically a small fee associated with sending Bitcoin transactions. This fee goes to the miners who verify and process the transactions on the Bitcoin network. The fee amount varies depending on the network traffic and the priority of the transaction.
As for testing the functionality of your wallet, there are some Bitcoin testnet faucets available that you can use to receive testnet Bitcoin for free. However, it's important to note that testnet Bitcoin is not real Bitcoin and cannot be used on the mainnet. It's purely for testing and experimentation purposes.
I hope that helps! Let me know if you have any more questions.