It is difficult to give an answer when you ask for the “best way” without defining “best way”. Not being sarcastic, How much $$$ do you want to invest? Encrypt all data or selected tables and or fields?
If you have an I5 (AS400 iSeries), then encryption capabilities do come with the OS.
There are many packages available to assist in encryption.
You must first define you objective. Encryption on backups? Show selected digits of account/SSN; to who and when? etc..