Truth tables give us a way to track logic functions depending on how their values vary. The order of the truth table matters — from top to bottom it follows binary digits.
An easy way to fill in the table by hand is to start from the rightmost bit, and alternate with digits of 0 and 1, where the starting column is , the column to the left is , etc.
For an automatic generator, see this link. You should know how to do it by hand, though.
Characteristic table
For sequential circuits, we have similar tables called characteristic tables. They’re not called truth tables since the values of the outputs are not determined by the current values of the inputs.