is it necesarry to register a company/LLC/something else to start taking payments online?

No. The payments will simply be made to you, and any liability rests solely with you. In this case, you'd be operating as either an individual or as a Sole Proprietor business entity. You can take payments with Paypal, Stripe, Gumroad, and many others simply as yourself/Sole Proprietor. In the US, when asked by the processor for your Tax ID at registration, you'd simply use your SSN. This is required because of Know Your Customer (KYC) banking regulations (here, YOU are the customer of the bank and payment provider).