One of the first things to consider when starting up an organisation is whether the need or issue you perceive is already being addressed. Check the THCVS Directory and Charity Commission search page to find out what other organisations exist in your area.
Overview of legal structures
This short guide from THCVS (Jan 2016) summarises some of the key legal steps and obligations that you will need to understand if you decide to set up a new community group, voluntary organisation or social enterprise.
The 'Starting Up' page from VolResource provides a very helpful set of questions for potential founders to consider before starting something new.
The Setting up a Charity wiki from KnowHow NonProfit is a huge, clear, and ever-expanding resource containing links to a range of different pages. Use it to get an overview of how charities and other socially motivated organisations are run and dip back into it as you and your team start to narrow down your ideas.
KnowHow NonProfit also has some great 'How To' guides, including this one on How to Write a Governing Document
NCVO and Bates Wells Braithwaite's Get Legal is a decision making tool that can help you decide what legal structure is best for your organisation. You need to register to use it but the sign up process is quick and easy.
The Charity Commission has a whole suite of resources to help charities who are just getting started. Their introductory guide shows you the main steps you need to take, then you can read through some of the more detailed guidance as you find your trustees and start developing your ideas.
The Charity Commission guidance page on Choosing a Structure gives a detailed breakdown of all the legal forms your charity can take and the reasons for choosing between them, and their 'How to write your charity's governing document'
The Charity Commission also produces model governing documents for small charities and charitable associations, trusts and companies, each of which includes guidance notes to help you to use it.
This government guide to setting up a social enterprise runs through the organisational forms suitable for 'not for profit' organisations.
Changing the World - A Young Person's Guide to Social Entrepreneurship is a highly visual, creative booklet that can help you explore your ideas and motivations
This Comprehensive Guide to Developing Your Social Enterprise from UnLtd, Hefce, Simpacta and Red Ochre is designed for social entrepreneurs but is likely to be of interest to all sorts of people and organisations.