The Information Commissioner's office has produced a 'Guide to Data Protection' on the practical impact of the GDPR and the Data Protection Act 2018. It is aimed at small and medium businesses and provides guidance for data protection officers and anyone else who has responsibility for data protection.
It is set out helpfully into sections, with drop down menus so you can easily access the information you need. It has an introduction which sets out what data protection is and the basic principles, perfect for start-ups or those new to data protection roles. It includes detailed explanations of important principles such as 'processing' and 'controlling'. It also contains a guide to the GDPR which is long but very readable and contains helpful working examples.
For sole traders or very small businesses, there are separate resources which can be reached using the same link.
Find the guidance here: https://ico.org.uk/for-organisations/guide-to-data-protection/