Speak to an accountant. You don't have to use their services, however most will give you initial advice free - such as whether sole trader/partnership/limited company is the best way forward for you. They will also be able to advise the most tax efficient ways of removing money from a company (assuming you go down the company route).
Look into VAT - know the thresholds so you know when you need to register for it.
Be realistic about turnover, business plan & cashflow. Cashflow is most definitely King for start-ups.
Other than that....just do it. Don't be one of these people who reads loads about it - you'll learn best when stuffing up, making mistakes and just doing it.
Good luck
