The account needs to have an effective balance of at least 1000 ACTN. That is, it needs to have had a balance of 1000 ACTN (or more) over the last 1440 blocks / 24 hours.

This is needed because the forging algorithm depends on the stake of the user.

The account needs to have at least one outgoing transaction, also confirmed 1440 times.

The passphrase is the account's private key, a public key is obtained only after at least one outgoing transaction is executed and confirmed. This can be done in several ways, notably by sending 1 ACTN to your own account or someone else's, by sending a message using the arbitrary message feature or by registering an alias.

When these two conditions are met, a 1000 ACTN effective balance and public key published, the account is eligible to forge.

Note that forging will stop if the effective balance drops below 1000 ACTN.