Here's a bit of advice that I would have appreciated when I started the program.
-First of all, I wished someone would have told me what a shock the workload would be at first. I was a student who was easily able to pass my classes with flying colours because it did not challenge. On the other hand, with IB, you need to expect the homework that often accompanies every lesson and be prepared to dedicate your time to complete as much of it as needed to help you understand the lesson.
-Secondly, get used to managing your time properly as you'll have a hard time with IB if you don't know how to balance everything. Like I said, sometimes, the workload can be massive and you need to prepare accordingly. On the other hand, you still need your 'you' time so make sure to plan these times strategically. That being said, one of the worst things you can do for yourself is PROCRASTINATE. If you do so, you'll always find yourself in a stressed position that you could have easily avoided. I understand that some people work better under time pressure (including me), but please just do yourself a favour and manage your time properly.
-Lastly, look into the curriculum of your classes and the study guides available for you. This way, you have a really good idea of what you're getting yourself into. You could also take the opportunity to learn in advance or take more time to learn things that you think may cause you difficulty in the future so you don't fall behind.
Hope this helps and good luck :)