Once you have an object, you read and write its fields using a dot:
Car myCar = new Car();
myCar.color = "red";
myCar.speed = 60;
System.out.println(myCar.color);
The dot tells Java: go to the object that myCar points to, then access the color field. Each object has its own copy of every field, so setting myCar.color to "red" has no effect on any other Car object.
If you try dot notation on a variable that is null, Java throws a NullPointerException. Always make sure the object exists before accessing its fields.