An account needs to have a stake pool certificate before it can participate in stake delegation between stake pools. A delegation 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.
Once the certificate is registered you can use these steps to help you delegate stake.