Encryption of data going into a database

We are designing a DIY website development product for service SMEs. We need some of the data saved by the user or created by user's customers not to be visible to us - but only the user and the user's customers. We need someone to be able to guide how to encrypt the data to that the data saved can be viewed/updated only the user and the user's customers. Only a small part of the data needs to have encryption - that is, most of the data will be unencrypted. The encryption is needed for a LOCKBOX where the user can store all their documents so that they can be conveniently recalled from a single place and the other is the user's customers' login/registration and the user's customers' dashboard containing chat as well as data like invoices, payments as well as other documents like blood test results and prescriptions in the case of medical practitioners; architectural drawings as in the case of an architect's office etc. that should be privy only to the user and the user's customers and not to the owner of the site.