If you handle peoples’ personal information (e.g. taking their contact details and recording them) you have a responsibility to look after that information in order to protect the person’s privacy. You should only keep the information you need for a specified purpose and only for as long as required for that purpose, and you should keep that information safe.
Where to go for information
Information Commissioner’s Office (ICO): have a comprehensive website, particularly their Guide to the General Data Protection Regulation (GDPR).