Backend of e-commerce

Hello all! 🙋🏻‍♂️ I’m Antonio from 🇪🇸 but based in 🇬🇧. I’m an artist and I want to start to selling my art online👨🏻‍🎨🤑🎉 . To do that I’m developing a website in Squarespace 👨🏻‍💻 . That’s so nice this company right? 👏🏼 , I almost finished the design of my website 😌 . And now begins the complicated part to me 😰 , I really don’t know how works the backend part 🤯 . I don’t know what plan I have to choose , I don’t know about taxes in uk, I don’t know how works the payments 😔 . YES! I’ve read it already, but still confused, a lot information isn’t? 🤦🏻‍♂️ Maybe it's just me, I’m hard to understand it idk 🤷🏻‍♂️, or I just need to be completly sure that I do everything good, I don't want to have legal problems 👨🏻‍⚖️ , much less misunderstandings with future customers 🤞🏼. 

Is there someone... in Squarespace... who has made an operational and legal online store? 👀 Can you help me please? 🙄 Thank to you heroes! 🔥🦸🏻‍♂️🦸🏻‍♀️🔥