Have you played Sealed before?
For the dragon's maze prerelease you're going to pick 1 of the 10 Ravnica guilds. You'll get a special pack in your box containing cards for that guild only. Then you'll get a another pack that has only cards for a random guild that shares 1 color with that guild. Then you'll get 4 regular Dragon's Maze booster packs.
You will then open those and build the best deck you can out of all the available cards. You don't have to play the guild you picked but you probably will. Your deck will be at least 3 colors. You need to build a deck of 40 cards, generally 23 cards and 17 lands (basic lands will be available to add to your deck up to the number you need.) You can alter your deck as much as you want between games and rounds if you don't like the choices you made.
This will be a difficult prerelease to build decks for. 3 colors makes it hard to know how many lands of each color to play. Some decks will be 4 or even 5 colors depending what cards get opened. You may want to ask for help building your deck.
You will play best-of-3 matches, at least 4 of them. Your first round you will get randomly paired against someone, it could be another new player or a regular. After that, pairings will be by standing in the event.
It's important to take your losses in good spirits.