Some observations from my experience:
Many people seem to have trouble with the introductory physics and calculus courses. Electromagnetism is a class that is often viewed as a weed-out class at my university.
At my school, there are a couple introductory engineering courses required. They are not difficult in subject matter, but they require a lot of work. I suspect they are designed to weed out the students who can't handle or are unwilling to spend the time required for upper level engineering courses.
As for preparation, take as much math and physics as you can. If you're in the US, try and take AP Calculus BC and AP Physics C. These will put you in a good position for your first year courses. (I don't know if there are similar programs outside of the US.) I would recommend not using AP credit and taking your first year math and physics courses. They usually cover more material and in more depth. They'll also form the basis for most of everything you will do during your undergrad, so you'll want to be strong in them.
Be prepared to spend a lot of time and put in a lot of work. It's challenging, but it's pretty fun.
If, when you take your first year courses, you find yourself having a hard time, remember there are a lot of resources available. Talk to your professors and attend review sessions. Some professional societies/honor societies also offer free tutoring. And of course, there's always PF!