Brian here from the Shopify Guru team. :) I hope you're having a lovely week.
We would always recommend getting in touch with your local tax authority or an account with regards to how you should set up your taxes and what you should be paying, as ultimately they'll have the most precise information for your case. As for actually opening a Shopify store, anyone can create them, but it's up to you, the merchant, to ensure that they are abiding by the tax regulations in their country.
Nonetheless though, I've taken a look around at what's required to set up a business in the UK, and gov.uk actually have a wonderful step by step process with explanations on each point that would be well worth taking a look at. You can find it here: https://www.gov.uk/topic/business-tax/self-employed
So first off it seems you'd want to get up as sole trader and register with HMRC, then get yourself registered for Self Assessment. As for VAT it seems that once your businesses is turning over more than £82,000, you'll have to register for a VAT registration certificate from HMRC.
Give the gov.uk guide above a read through, as it should answer quite a few questions, and for more specific queries related to your case, HMRC or a local accountant should be assist and ensure you get the correct information. :)
I hope this helps, and if you have any other questions or issues, please do not hesitate to get in touch with us at email@example.com.
Have a lovely day!