I can't execute a send transaction method of my smart contract from the frontend by Alchemy

I am a beginner and I am trying to execute a method from my frontend that sends a transaction to the blockchain but I get an error from alchemy saying that the transaction needs to be signed with my private key but I have no idea how to do this and I did not find anything similar In Internet

Error in the front: Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'transactionHash')

Error in Alchemy: Unsupported method: eth_sendTransaction. Alchemy does not hold users' private keys. See available methods at https://docs.alchemy.com/alchemy/documentation/apis

Error in Alchemy: { "jsonrpc": "2.0", "error": { "code": -32600, "message": "Unsupported method: eth_sendTransaction. Alchemy does not hold users' private keys. See available methods at https://docs.alchemy.com/alchemy/documentation/apis" }, "id": 4 }