Since the highest number you're counting to, which is 8, translates to 1000, you would need 4 bits to express your outputs.
Your truth table (aka transition table in this case) will have one column for current state and one column for next state. I would also add another column for the flip flop inputs. For the current state, you would just list your numbers from top to bottom in the order you would like them to be. I.e. 0,4,8,2,6. Then the next state column would just be the numbers that come after each current state. For example, the next state after 6 would be 0 since you want to cycle through; next state after 0 would be 4 etc etc.
From that, you obtain the current state and next state transitions for each bit. For instance, the first entry, transition from 0000 to 0100, if you are looking at the second most significant bit, the transition would be from current state to next state would be 0 to 1. The excitation table of whichever flip flop you're using will tell you what this translates to as far as the flip flop input is concerned. For a D flip flop, that transition would give a D input of 1. You do this for every bit in a row. For instance, the first row, 0000 to 0100, if you're using the D flip flop, the truth table entry for the D column would be 0100 based on the transition of each bit.
Just out of curiosity, have you done state machine design much in your class?