The mail that you get will contain the invoice with details of your package: resort details (type of room booked), type of park tickets, type of dining plan.
It will also have a bunch of coupons for things like mini golf, boat rentals, arcade cards. And it will have a couple of WDW luggage tags.
If you have signed up for Magic Express you will get a separate mail with your ME luggage tags.
Since you have signed up for the dining plan, if it is the
DDP (not the QSDP) you need to book your Table Service ADRs (Reservations). And you need to do it soon, since this (in Disney terms) is a last minute reservation. You may have difficulty getting your first choices, since most WDW vets do this around 180 days out.
You should have figured out which park you will do on each day of your visit first, to make everything more time efficient. In other words, make all of your ADRs for restaurants in and around the MK on a MK day. A major waste of time to plan an Epcot day, but have a breakfast planned at AKL.
You can check in online if you prefer.
When you arrive at the hotel, give them the documents showing your resort confirmation number and proof of payment (the invoice from your mail). You will be given the KTTW card for each person in your party. This is your room key card, park entry card, dining plan payment, and allows charging privileges to your room if you have left a credit card number at check in for this purpose.
Hope this helped.