First step is look at the Charity Commission website. All the information you need initially is on there. Formation of the charity - can do it yourself if not complex or you can use a firm to do it for you (don't use general solicitors or accountants etc used to setting up businesses).
Main points Can't be set up to benefit one specific individual Needs to provide a 'public benefit' To be a registered charity you need income of over £5k a year - unless it is a CIO (Charitable Incorporated Organisation) when you can register with any income. Different structures of charities: trusts, Charitable Companies, CIOs. We tend to recommend CIOs to most people now.
You can raise funds before registration but must make it clear you are unregistered charity, and once you have £5k it is compulsory to register.