Ability Checks

d20¹  +  ability modifier²  +  proficiency bonus³  +  other bonus or penalty   ≥   DC of task

¹ Roll 2d20 if advantage or disadvantage applies and use the appropriate result. Go here to see the various reasons why either would apply.

² The GM decides which of the six abilities is relevant to the task at hand. The corresponding ability modifier is used.

³ Add your proficiency bonus if a skill applies and you are proficient in it or a tool is necessary for the task and you are proficient with the tool. 
See also: Variant: Skills with Different Abilities.