To set up a limited company, you first have to apply to Companies House to have your organization officially recognized. For this you have to fill in a form which can be downloaded from the internet. You also need to complete a memorandum of association, which lists all the company members and confirms that they have agreed to join the company. The articles of assocation is a further document that explains in detail about shareholders' rights, directors duties and powers and other legally required information.

It is possible to carry out these and all the other tasks yourself, but most people choose to employ someone to complete the incorporation (process of setting up a limited company) as it is very complex and there are a great many legal and other issues to sort out. You can find advice, as well as plenty of links to other sources of information, here.