In the US, each public transportation system has its own rules about masks. Hubs like airports and train stations have their own requirements, too, as do air, rail, bus, and cruise carriers. Businesses, public buildings, and other venues may also have masking rules you'll need to follow, as will local health agencies. You can find your local health agency here.
Otherwise, if you’re up to date on vaccines, you only need to wear a mask if there’s a lot of COVID in your area or if your immune system doesn't work well. If that’s the case:
Wear a mask when you’re indoors with people you don’t live with.
Outdoors, consider masking if you’re in a crowd or if you’ll be near people who aren’t vaccinated.
And remember, if you feel more comfortable wearing a mask in public places, you can always choose to wear one.