It's been too long, so I won't address the contents of the packet too specifically. Generally, however, you will receive a notification of what time to show up, what records (if any) to bring, what to bring, and what NOT to bring.
As to what to bring, I-Day is one of those days where you can quite literally show up stark naked and be fully equipped for Plebe Summer by the end of the day. They will issue you everything you need, right down to toiletries and underwear.
Don't bring anything you're not willing to haul around all day. Don't bring anything that requires a bag to carry. I know people who showed up in jeans, sneakers, a T-shirt, their wallet, their orders, and an Ultra-Brite smile (which didn't last long). If you have prescription meds, bring enough for a week or two, and have your parents send you the remainder. You can always go to Medical for them based upon your documented condition, but that may take a few days until you have the "free time" to do so. I strongly suspect your admission packet will address this in better detail.
That's all that occurs to me right now. Just remember, they will issue you EVERYTHING you need.