Not sure which tax you are talking about as you dont specify, but assuming you are talking about a store in the UK, the following would apply:
Tax would be payable on profits, either self assessment income tax and NI if you are a sole trader or corporation tax if it's a company.
VAT would be calculated on taxable turnover less vat incurred on purchases, but only if you are VAT registered.
If you employ people also need to consider PAYE and NI.
If you could give us more info on the retail business, do you trade online, sell to overseas customers, company or sole trader, etc, we would be able to provide more tailored advice including deadlines.
I would.however suggest you talk through your specific questions with an accountant as it is betterto get the right advice up front before you get yourself into a mess!
