It is all part of doing your taxes. If your business has a revenue in excess of about £50k you must charge VAT of 20% to your customers and then pass that onto the tax man. You must also pay tax on your personal earnings and tax on the business's profits. There are all sorts of glamorous ways to get around much of this. I suggest you start reading up on it.
To start with, you probably want to set yourself up as a single trader, but as you grow and start to look for premises and other business expenses start to rack up, creating a limited company might be the way to go.
Oh - and don't forget your public liability insurance! You might also consider some form of insurance for if you seriously screw up someone's prized phone and they try to sue you.
Good luck!