When you start employing people then you need to have a basic understanding of employment law. ACAS is a good source of information about employment law, and they have a free telephone helpline for all employers: Contact us | Acas. You must also buy employers liability insurance, which is a legal requirement.
You will need to decide and/or determine whether the people you pay to work for your organisation are self-employed or employees. If you take on staff then you will be responsible for calculating and paying their tax and National Insurance contributions under PAYE (Pay As You Earn). You can find out more information about this here. There are penalties for dealing with tax obligations incorrectly or for paying someone ‘cash in hand’ without properly determining that they are paying their own tax and National Insurance. Many organisations therefore work with accountants and/or payroll providers to help them to manage the process of paying staff. You should always research providers and seek quotations to make sure you get a service that is suitable for your needs.