This needs expanding, and could be expanded into a number of degree level courses.

There are a number of legal structure options. The most popular are:

• unincorporated organisation – a bunch of people doing stuff. How most campaigning groups start out. Members are totally liable for debts and for the crimes of each other. Campaign Against the Arms Trade is one of these. So is War Resisters International (although it comprises not people but organisations).
• company limited by guarantee. Like Conscience. Protects the founders to a large extent should someone do a Bad Thing. Means you can’t break the law.
• Community Interest Company. Created for the good of a community and not suitable for campaigning. Meant for allotment associations or a village school run by the residents.
• Charity. Very heavily regulated and controlled. Cannot get involved in political campaigning (in theory – the rich ones do it all the time).

The above are listed on gov.uk which has really good information on the options.

Regarding the purpose, vision and mission statement, you need to look inside your heart for those. They must be expressed in a way that is understandable by others and are goals that could be achieved. It might be one thing, such as COMT (like Conscience) or wider ranging such as campaigning for people being harmed by their government (similar to Amnesty International). It can be hugely wide, like the World Wildlife Fund, or as narrow as the Faslane Peace Camp.

When someone says “Who are you?”, “What are you campaigning for?”, “Why does it matter?”, “Why should I care?”, “What do you expect me to do about it?” those answers should be there already. (We ought to have a page of those on the Conscience web site.) It is worth coming up with a list of those questions and keeping them somewhere to hand and as answers occur to you, write them down. If you can’t answer them, you can’t get others to follow. It might mean changing the purpose, narrowing it down, until you can come up with questions and answers that work.