Everyone's exam will be different because the computer will adapt to each individual, if the exam is the NREMT-P exam. Yes, it will be as difficult as the limit of your knowledge. In short, at some point, you'll get 50% of the questions correct. Once you've satisfied a pass/fail rule, the computer will shut off and you'll leave the test center without knowing if you passed or failed.
Now then, since everyone's exams will be different, your best source for what to study is contained in an approved course. The only advice I could give (really) about NREMT (or for that matter, NCLEX) is that the testing world bears little resemblance to reality and is its own "ivory tower" world.