1St Oil Change ,what should I know before I go?

It's important to use the recommended grade and weight, and to not overfill at all. See the owner's manual. To be very safe, tell them to put in exactly three quarts and that you'll top it off yourself later (three quarts fills the system including the filter exactly halfway, which is ideal, so you actually won't have to "top it off"). Also get them to agree that you'll check it yourself on their drive and if it's overfilled by as much as a millimeter they will drain it down. Remember to turn it OFF and wait 15 minutes to get an accurate reading.

I use a dealer because they *should* at least have someone who knows how to drive a Prius. I don't trust handing a "How To Drive Your Prius" instruction card to the average grease monkey.