I am assuming you eat breakfast before you bike. If not, start eating breakfast.
Other than that I think that you just need to keep riding and build up your endurance.
At 8 km, you should have no special nutrition needs, as I am also assuming this takes you 20-30 minutes.
If you feel hungry arriving at work (assuming you ate breakfast of course), a small snack would be fine - an apple, banana, or granola bar of some type would do the trick.