@fdanielix I went to the last D23 Expo in 2013, so this will be my second one.
If you preordered your ticket earlier, it will be mailed to you (mine arrived today), but if you ordered it later you may need to pick it up the night before.
There are two lines in the beginning of the day: one for D23 members and one for non-members. Additionally, there may be a separate line for a morning presentation. So make sure you get in the correct line! You'll want to be out early, and I recommend eating breakfast in line if you can.
Be prepared to line up for anything you want to see up to several hours ahead. A lot of people sit on the floor and wait, so you may want to consider something cushy to sit on if you'll be there all 3 days. Also, there are limited plugs available for charging your phone, so keep that in mind if you want to use your phone while you wait in line
Definitely pick up a guide book! There was also an app last time that was very helpful. There are a lot of cool things to see, but they often overlap (I suspect that's for crowd control), so you'll want to remain flexible on any plans.
Certain events have a "fast-pass" where you can reserve a spot, but you can only get so many passes per day, so be sure to check online/at the special desk to be sure of what's available.
The temperature was pretty good outside and inside of the convention center, but I recommend bringing a lightweight jacket. Comfy shoes and clothes are a must!
There will be a lot of freebies to grab, and last time they had free totes for carting everything around in. USPS is also supposed to have a booth there if you need to mail any goodies back home.
Some presentations (like the animation one in Hall D23 with John Lasseter) require that you silence your phone and put it in a special bag. You can keep any device that fits in a bag on your person, but large video/photo equipment had to be left behind. I only came across one panel that this happened on, but be prepared for it.
There are two food courts within the convention center, in the center back on the left and right sides (I found the right side to be less crowded/noisy) and food cannot leave the court. There are also good food options in the basement of the Hilton across the pathway, and the Marriott (to the right) supposedly has a Pizza Hut and Starbucks.
There are some Disney celebrities roaming the floor and at booths. Last time I saw Bill Farmer (Goofy), Pat Carroll (Ursula), Ridley Pearson (author), Steven Clark (big cheese at D23), and more. Some ask for money to get an autograph/photo, others do it for free.
Do you mind if I ask how far you are coming in from? (Train, bus, flight, car, etc)
The ART bus (Anaheim Resort Transit) has a lot of routes in the area, including stops at the convention center, nearby hotels, Disneyland parks and Downtown Disney, and the new ARTIC station. Passes are available online and in Anaheim (when I was there they didn't accept cash at the bus, so make sure you get a pass if you plan to ride). The walk from the convention center to Downtown Disney or the Disney hotels can be tiring after walking all day at the Expo, so you may want to consider getting a pass (it's cheaper than a taxi).
Most people are pretty nice, and you can make friends anywhere. And of course, HAVE FUN!!
Let me know if you have any other questions.