It's probably your genes, in which case just simple ~korean skin care~ won't help.
Skin care/lifestyle/styling/make up: >>86386
what this anon said. Sun screen is your friend. If you have oily skin, make sure not to get super creamy moisturizer because it'll make your pores look huge.
You should also look up on pintrest or insta or an image board pics of a young/young seeming girl so you have a sense of style. For example, I love Ariana Grande and look quite similar to her. She's 25 and looks 10 years younger - why? First of all, she's very petite. But more importantly, her style keeps her looking very young and sweet and innocent. Also, eyebrows - they're very important. Super arched brows will make you look older. Straight, slightly arched at the ends will make you look younger.
Now, you're probably lacking in fat in parts of your face and probably have an issue with wrinkles or something. If you have deep eye hollows, you should get fillers. They're worth it, trust me. Naturally placed cheek and lip fillers go a long ass way too. Probably the worst thing dragging you down is 1) your skin and then 2) your style, so you need to work on both of those.