How do you use DC when you cast a spell [duplicate]

This question already has an answer here:

  • How to determine the result of a saving throw?

    2 answers

Suppose I'm using a spell that requires the target to do a DEX backup or take 2d6 fire damage. What would I compare to the creatures that DEX records? Example: cast a spell saying, "Target creature makes a DEX save roll, if it fails, take 2d6 fire". What would I compare DEX roll targets to?

Sorry guys for the trouble, I'm pretty new to D & D