Every ruling is really two decisions. Should this be rolled for, and if so, against what number. New DMs reach straight for the second one and skip the first, which is the one that actually matters.
Most actions just happen. Call for a roll only when failure is genuinely possible and failing would cost something interesting — if either half is missing, say what happens and move on. Rolling for everything makes competent characters look incompetent, because a d20 says they fail a fifth of the time even at what they are trained for. And calling for a roll you will not honour — narrating success anyway once the dice land wrong — teaches the table that rolls do not matter, which is worse than not calling for one at all.
Ability checks are the thing being decided here. If you would not be surprised by either outcome, it is a roll. If you would only ever accept one outcome, it is not.
Once you have decided a roll matters, the target is a Difficulty Class. Pick it before you see the roll, not after, and use the table below rather than inventing a number in the moment.
| Task | DC |
|---|---|
| Very easy | 5 |
| Easy | 10 |
| Medium | 15 |
| Hard | 20 |
| Very hard | 25 |
| Nearly impossible | 30 |
Most of what you set lives at 10, 15 or 20. Reaching past those three should feel like a decision, not a habit. A DC 5 is usually a thing that should just succeed without a roll at all; a DC 30 is usually a thing that should just fail, reserved for the rare moment where succeeding at it would be the story.
The lever most new DMs do not reach for. When a player's idea makes the attempt genuinely easier or harder, resist the urge to shift the DC by five and do the arithmetic mid-scene. Express it as advantage or disadvantage instead — a good idea changes the odds, and the dice can carry that on their own. Circumstance moves the dice; the task itself is what sets the DC.
Not everything worth noticing gets a roll, because not everyone thinks to ask for one. A character's passive perception, and the passive scores generally, tell you what they notice without anyone saying they are looking. Compare the passive score against the DC the same way you would a rolled check, no dice involved. This is what stops the table having to announce a search of every corridor to find anything in it.
Worth its own heading, because DMs conflate the two. A check is someone trying to do something; a saving throw is someone resisting something that has already happened to them. The difference decides who rolls and whether the number you are comparing against is the DC you just set or a fixed save DC. For the mechanics of both, see D20 Tests.