Very helpful and complete information about travel to/in and within the USA can be found on the website of the CDC via this link including advices on what to do or not do when travelling.
The US Department of State gives some general information here for the whole USA and here you will find most of the FAQ about travel in/to and within the USA.
For travel restrictions due to COVID-19 in Europe, Canada or the rest of the world, see our specific FAQ about Europe, Canada or the rest of the world