When I started commuting a couple of years ago, I hadn't been on a bike in years and I have a lot of hills in my area. I pretty much did what others mentioned above. I rode everyday and stopped to rest on the hills when I needed to. I don't recommend walking up hills. Try resting for 3-5 minutes and have another whack at it. It may seem like it takes longer to get home, but I think you'll get into condition faster this way. I also rode on my days off, taking it easy if I was tired or sore. Sometimes it's better to take an easy, "recovery" ride than a rest day.