1. You will lose sleep at some point

I'm just going to say this at the outset. It happens.

2. IB is not like your average class

In schools that are not specifically designed for IB, your IB classes are almost like two classes in one. Not only are they sometimes combined with AP students at public high schools (this has gotten better) but you really do receive two grades: your class grade (the one that goes on your transcript and is counted in GPA) and your IB grade (which is based on your IA, IB test, and whatever other IB assignments you have)

3. Acronyms are pretty much everywhere

IB loves it's acronyms and pretty much every IB assignment you do will have one. To save later confusion, here is what they all mean.
IA: Internal assessment. This is a paper you write that is still graded by your teachers but is moderated by the IBO. This is the most common IB assignment.
EA: External assessment. I've only ever seen this one in language classes. It's pretty much like an IA, only graded by someone in a far away land.
EE: Extended essay. This is a 4,000 word research paper that you write between junior and senior year on pretty much anything you want. I wrote mine on globalization in Nigeria and my friend Annie wrote hers on how black holes are depicted in literature; they can be literally anything.
IOP: individual oral presentation. This is a kind of "research literature presentation" as I like to call it that you do your junior year in English class. It can also be pretty much on anything you want.
IOC: individual oral commentary. This is a literary commentary done your senior year in English. It is much less flexible than the IOP and can only be done on a passage you pull from an envelope thirty minutes before the commentary starts. Have fun!
TOK: theory of knowledge. This class is so weird. You take it second semester of junior year and first semester of senior year. There are two IB assignments, a presentation and an essay in the first semester and second semester respectively.
CAS: creativity, action, service. The bane of an IB students existence, this acronym is a fancy way of making you be productive outside the classroom and keep track of it.
TWE: to what extent. This phrase is everywhere

4. "To what extent" is your new best friend...

Nearly everything you want to ask can be phrased this way (and will be phrased this way) in order to ask weird questions you wouldn't normally think about. It's pretty useful for having discussions of great nuance.

5. ...and your new worst enemy

Every time you make a good point and think to yourself, "damn look at me making such good points," some person two rows behind you will say, "yeah but to what extent?" Don't be too mad at them though, I'm pretty sure everyone in IB has done this at least once. Sometimes it is almost a reflex.

6. Literally nothing is as simple as it seems at first glance.

IB will force you to delve deeper into subjects than you have ever gone before. This is true of nearly every content area. Everything now has an asterisk attached to it and everything can be parsed down into finer details. This is especially true in the social sciences where your class will be nearly all discussion based.

7. Cross discipline learning is a real thing

IB is intended to be a coordinated program across multiple content areas. So what you are learning in history or English might very well apply to a psychology course or vice versa. Your physics class might have a section that is very similar to your bio class. Be prepared to think over an issue from multiple perspectives.

8. You'd better get used to seeing a lot of the same people

In IB, most of your classes will be with the same core group of students. Even as the program has gotten larger at many schools it is still not uncommon to have 4 or 5 classes with a number of people. My junior year I had 5 classes where 15 or so of the people were the same. My sister and her best friend have the exact same schedule this year (teachers beware of these two). There's really no avoiding this so if you've got a problem with someone, sorting it out is your best option.

9. Keep the IB family

IB was a really close knit community of students during my time in the program. From having many of the same classes to having late night study sessions of 20 people in the basement of Honnold, we all pretty stuck together through our two years. Don't discount the value of having people there who will help you out when you're totally lost.

10. Ending up totally lost is not a big deal

This has happened to pretty much everyone in the program. A combination of long days and long nights has a tendency to fry your brain sometimes. However, your teachers and classmates are really understanding about this because they've gone through it and seen it themselves. Don't be afraid to talk to them if you need to.

11. If you are a perfectionist-don't be

Do your work well of course and don't slack off, but recognize that not every assignment has to be a work of art. Notes on 30 pages of history will go by a lot more quickly if you don't try to write down every single thing that might be just a little important down the line.

12. Literally don't even worry about it

There are so many stories out there about IB that it can be hard to separate things that have actually happened from things we've made up just to scare people. I'm not saying that IB is going to be a walk in the park because it won't be, but worrying about everything is not going to make it any easier. More likely than not, everything will turn out fine.