Private DM rolls

this may have been adressed a million time but i cant find an answer and ive tried to follow instructions. i like to keep my rolls private from my players and i know i can change once game is launched but i forgot to do it sometimes and my player wont tell me lol. is there a way to have it default to always rolling privately to myself? please tell me like im an idiot because ive tried to follow instructions ive found online and cant figure it out. 

The answer depends on what game system you are playing and also on what character sheet you are using for that system. 

I don't think there's a default method ie a VTT setting, but you might try  /talktomyself  and all of your rolls will be hidden from others in the game.  Great for testing things out between sessions or working out a custom macro without flooding chat. One down side is that API commands won't work when enabled.

Also, if you are rolling using the dice roller, you can set that to whisper by default.

Or, if seeing the dice isn't a deal breaker, you can just whisper an inline roll to yourself: /w gm [[2d20kh1]]

One more thing, are you talking about just rolling dice in general, or are you talking about the default whisper settings for your NPCs? If it's the latter, we'll need to know exactly which character sheet you are using (D&D 5e (2014), by Roll20, Pathfinder 2 by Roll20) , etc. .