In the broad sense, when playing you just say what you want to do and your character can do it.  When there is a chance of failure and consequences (like when fighting), then you’ll roll a 20-sided die, trying to beat a target number set by the dungeon master, and adding a bonus to that roll when your character is skilled at that task: fighting, magic, sneaking, crafting, etc.  There are the rule books for making a character, plus some free resources with the basics, like a System Reference Document: https://5thsrd.org/