If you are in business, you must register for VAT if your turnover for the previous 12 months is above £73,000.
This is known as the VAT registration threshold. It changes regularly, so you should regularly check your turnover against the current threshold.
You must also register for VAT if:
The HMRC guidance on how to do this is here:
Guide on InvoicingThis must be adhered to otherwise the vat could end up coming from your sales value.
When you register you can look back over your purchases and in some circumstances you can reclaim vat:
There are certain conditions which need to be met to be able to do this so have a look at the full guidance which can be found here.
Note these conditions in particular:
You can reclaim VAT on goods you bought or imported no more than three years before you were registered for VAT if the goods:
You can't reclaim VAT on goods that you've completely used up before you registered for VAT such as petrol, electricity or gas.
So for items already sold onto customers you cannot claim the vat.