If you are a new company that wishes to offer shares to the public, then you will be beginning the process of an IPO. It is important to understand all of the legal and financial regulations that surround this before you begin selling shares left right and centre.
In the first instance, it is important to speak with an accountant to ensure that your business financial records are up to date as it is likely you will have to publish these.
Your business must also be a limited company or partnership in order to sell shares. If you are a sole trader and not registered with Companies House then you will not be able to sell your shares to the public. Once you are registered with Companies House you will be given the option to then sell shares as a company, or in other words start an IPO.