This site contains all the info you need to prepare for your adventure. Start with the "Story Synopsis" below, then read as much of the rest as you like, but the "What You Need to Do to Prepare" section will probably be the most helpful. Clicking on the links will give you more information if you want it, but it's all optional.
You are a member of an isolated species known as the proteans. Your kind are natural shapeshifters, able to change into any form you can imagine, although more complicated forms can only be achieved by the elders of your people.
You are durlans, the youths of the proteans. This is not so much about physical age—your people do not age as other species do, because you can always just change to a younger form—but rather about relative inexperience. As durlans, you cannot change into anything smaller than a cat, or larger than a horse. You can change into anything that is natural and solid (such as an animal, plant, or rock), and you can change into anything man-made, but it will only be functional if you personally know how to make that thing. So you can turn into a chair, and people can sit on you, but if you try to turn into a telescope, people won't be able to use you to see far away things.
The elders of the proteans are known as morphs, and they can turn into much smaller or much larger forms, plus they have access to a wider variety of functional man-made objects (as their greater experience has taught them how more such things function). The truly ancient proteans are called the eremain, and they can even take on non-solid forms, such as fire, wind, or clouds. The eremain are worshipped by some members of other species as powerful nature spirits, but to you and your fellow durlans, they are just wise people who offer advice, wisdom, and, sometimes ... a mission.
Your job, along with your fellow party members, is to go on missions assigned to you by the morphs, or occasionally by the eremain. The proteans are reclusive, but they try to stamp out evil where they find it, believing that helping others helps everyone thrive. You have worked together for some time, so you all know each other pretty well, and you have some experience under your belt. As our story begins, you are summoned to appear before Inari, a powerful eremain ...
Hi, I'm Buddy, Perrin's father. I'll be your DM, or Dungeon Master. It will be my job to keep the story moving, introduce the non-player characters, run the monsters, and generally be the referee.
There's lots of different ways to play D&D, and lots of different styles of DMing. Just to let you know, I favor a story-focussed approach, where the goal is to tell a really cool story that you will remember for years to come. Especially since this is a one-shot (that is, a self-contained adventure designed to be run in one day), you don't have to worry about anything too bad happening to your character. We're just going to mess around and have fun with it.
If you know the rules of D&D, you may be interested to know about the house rules we use.
All you need to do is come up with an idea for a character. If you want to flesh that out, read through the "Character Creation" section below; it's especially useful if you already have some experience making D&D characters.
If you don't have experience, that's okay: just come up with an idea of what sort of character you'd like to play—someone who's good at fighting, someone who's good at magic, someone who's good at sneaking—and I can help you figure out the rest. You do not have to learn any of the rules of D&D before we start playing. You can learn as you go, and it won't be so overwhelming as if you try to learn everything beforehand.
You can work on your character on your own, with me, or with the other players. Talking through your ideas in a group can be lots of fun, and helps everyone come up with complementary characters.
If you have dice, feel free to bring them, but you don't have to.
If you want to start filling out your character sheet ahead of time, you can use the simple fillable character sheet from Wizards, or you can try using Myth Weavers. Myth Weavers has sheets that will do some of the math for you, but you have to sign up for a (free) account.
If you're experienced with D&D, feel free to make your character yourself. If you're new to it, don't worry: I will work with you to create something that you'll enjoy playing. Or you can take a middle approach: fill out as much of your character sheet as you want, and I'll help you with the rest. Whatever is most comfortable for you.
Although your species is set for you, because you are a shapeshifter you can feel free to look like any other classic D&D humanoid that you like: human, elf, dwarf, halfling, dragonborn, tiefling, etc. Or any creature that isn't from D&D. Or anything you can imagine, as long as you stay with the size guidelines (roughly, no smaller than a cat and no larger than a horse).
If you want to read the detailed character creation guidelines, you can; they will make the most sense if you know at least a little about D&D.