Welcome to AU! Wonderful trip!
Please read the Forum that has tips for First Time Riders and the Trip Report Forums, lots of good info!
As for your Coach trip, of course you'll want to wear layers of comfortable Clothing and comfortable Footwear( Sneakers work well), take a travel pillow and light blanket or throw,.
You can keep your Big luggage downstairs in the Luggage rack and just take what you need upstairs to your Seat.
Most of us like to sit Upstairs in Superliners since the egress through the Train to the Cafe and Sightseer Lounge is upstairs and the View is better.
Downstairs is where the Restrooms are, and Amtrak tries to save these seats for the Mobility impaired and Seniors.
The best ride is in the Middle of the car( not by the Stairs) and Window Seats give you the best Asfor your View of the passing scenery and also make it easier to clean against the wall to sleep if you have yo share a Seat with another passenger.
As for your food and drink, you can carry on food and drink, and the Cafe attendant will give you a cup.of hot water upon request.
Amtrak currently sells Dunkin Coffee @ $ 2.75 a cup, it's not bad, but is bestcwhdn freshly brewed in the morning for Breskfast.
The National Menu in the Cafes and Diner are changing once Full Service returns this Summer( Daily Trains) so be sure and read them once they are Posted.