Congratulations Andrew! You'll love your Dexcom.
As for showering, just leave everything alone and continue as normal. The receiver (bigger unit where you read your blood sugar numbers) is NOT waterproof so keep it clear of any water, but otherwise just leave your "site" alone and you'll be fine.
You might consider picking up some OpSite to put over your "site". It will keep it on your body longer and drier.