When fulfilling your duties as a landlord, you will inevitably come into contact with your tenant's personal information, for example, their email addresses, passport number, etc. You will have a need to process that information when you do things such as keeping a copy of the tenancy agreement (that lists their personal information) or a copy of their passport to comply with right to rent legislation.
Under the Data Protection Act 2018, and the UK General Data Protection Regulation, most organisations that process personal information must register with the Information Commissioner's Office (ICO).
There are some exemptions, but as a landlord, this is likely to apply to you.
The ICO has a quick assessment guide to find out if you need to register.
If you do need to register, there might be a fee involved, which will vary depending on the size of the organisation and turnover.
The ICO has a fee-assessment tool to help you determine if you need to pay a fee.
See the Information Commissioner's website for more details.