For a beginner with an 8 mile commute, I'd say:
1. Carry extra clothes. Or if you have place in your office, keep a set of office clothes next time you drive in. You are going to get sweaty, be prepared to clean up and change. If you sit in sweaty clothes for the rest of the day, you will not like it and might even stop cycling.
2. Route selection: look for bike friendly roads (even if they add a little to your commute).
4. Have a backup plan in case of mechanical issues or a flat tire - cab or friend, or just buy a repair kit and see a couple of videos online. A kit will set you back $8 (tire levers and patches) and a portable pump will cost $20 - $30. Its easy enough to do, if you've seen a video and have the right tools.