An account needs to have a stake pool certificate before it can participate in stake delegation between stake pools. This certificate is a cryptographically-signed piece of metadata in the blockchain that transfers staking rights from one staking key to another. The certificate contains the staking-key of the pool leader and is sent to the blockchain to register the stake pool with the other participants of the blockchain. A delegation certificate is published on the blockchain, therefore transaction fees apply. When you are creating a stake pool, you need to generate a registration certificate and embed it in a special transaction that creates the stake pool. Please refer to these instructions on how to register your stake pool on the blockchain.
Once the certificate is registered on the blockchain a node ID is assigned to the stake pool. The second stage of registration is to register your stake pool owner metadata with the Cardano Foundation, please refer to Registering stake pools with the Cardano Foundation stake pool registry.