You just act out some stuff, and anytime the stuff you say pure going to do is not garunteed to work and also not def going to fail, you roll a 20 sided die. Then you add numbees to that, the higher your total the better you do. Usually there are 6 stats (strength, dexterity, constitution, intelligence, wisdom and charisma) depending on what you’re doing you’ll usually add a number based on one of those. If you’re really good at a thing in particular (proficient) you add more or that roll on top of the ability based number.
Beyond that, you decide what kind of character you want—essentially do you want a lot of physical prowess or are you more into magic and the mind? Maybe you want some of both? Your class helps shape all of that and what kinds of things you can do.
Next step would be to do a little google research or ask specific questions tho.