How do I buy alcohol for the first time?

Sorry this is probably a really stupid question but I’ve never bought alcohol (I’ve drank though) and am looking to buy wine tonight. I turned 21 yesterday so I know I shouldn’t be nervous but I get social anxiety and worry about not knowing the layout of the store and stuff and I know nothing about this so I’m stressed😭 and I don’t wanna overpay without realizing. Any advice?