Tonight is the first seder! If all goes as planned, you're going to have a great time! And if not, ur doin it wrong. (If you don't know this internet meme, check out this, this, and this. All are "safe for work," assuming you're supposed to be on the internet at all!)
In all seriousness, there is a reason why so many people would choose Pesach as their favorite holiday! If someone can say that with a straight face after cleaning and kashering for Pesach, there must be something there! I think it goes beyond the warm and fuzzy childhood memories that many people have about Pesach. And though that may be a significant influence, converts and BTs also get to build those memories with their friends and families. Of course, that takes time, and that knowledge may not help you now. Personally, I've only had seders with near-strangers, and I've never had anything but a great time. Four (or more) cups of wine certainly encourages people to relax!
Assuming you've never been to a seder before, let's talk about some of the major things you should expect to experience. Most parts of the seder are family traditions, and every family's seder is different.
As a threshold matter, realize that this is going to be a very long meal. And at least the first half won't have the actual meal! By the time the official meal arrives, you're going to be starving.
There will be an assigned "leader" of the seder. If you're like me, you're just glad it's not you! This person (usually the male head of the house) will have spent a significant amount of time preparing to lead the seder and to offer several Torah insights throughout the evening.
The best part? The seder comes with an "instruction" book: the hagaddah. It will literally spell out what you will do and what people will say. It's amazing for the clueless. Of course, some hagaddahs are better than others, and you make do with what you have.
You will almost certainly be expected to read out loud from the hagaddah. Some people may read their section in Hebrew. You're going to be just fine if you read in English. (And you can read along in English when others recite the Hebrew.)
Pay special attention to the Four Sons. Be like the wise son and ask good questions. And if you can't ask an informed question, be like the simple son and ask "What's that?" Of course, at least once, you will be like the son who doesn't know to ask. And that's ok. Just don't be the wicked son! The moral of the story? Questions are encouraged on Passover. Ask them. No need to feel silly; the seder is intended to be a structured learning exercise. Really.
Four cups of wine. For your sake, I hope you like the wine you get. If you are a recovering alcoholic or otherwise don't (or can't) consume wine, you will use grape juice or sparkling grape juice. The amount that fulfills the requirement of a "cup" is actually very small: about 3.2 ounces. If any of this would be a problem for you, speak with your rabbi for possible alternatives.
Yes, you have to eat matzo in order to fulfill several mitzvot (making hamotzi, eating matzah for Pesach, and the afikomen-perhaps afikomen is simply custom?). If you're like me, you hope the two seders are the only time you'll eat this intestinal shellac for another year. Of course, there are people who actually like matzah enough to eat it year-round. I am not one of those people.
There will probably be lots of singing. Just play along.
You'll be doing a lot of leaning to the left (physically, not politically). I feel silly when doing this without an armrest because it seems unnatural. Just lean and enjoy that the leaning signifies your freedom!
These are only some highlights of what you'll experience tonight and tomorrow night. May you learn a lot and have a wonderful time! And if you feel like you did it all wrong, don't worry! You get a second chance tomorrow night!
If you want to be a Seder Superstar, do some reading beforehand and come prepared with some short Torah insight about Passover.
Chag Pesach kasher v'sameach! Have a happy and kosher Passover! Next year in Jerusalem!
(As an interesting sidenote, the Pesach seder is one of the few time-bound mitzvot that women are obligated to do.)