The NET only has reading and math.
The math section is basic, and easy to most. (addition, subtraction, fractions, percentages, ratios, and basic algebra.)
The reading is the more difficult of the two. You have to read passages, come up with inferences of what could happen based on what you read, purpose of the writing, main idea, and predicting outcomes. The difficult thing is that it is timed, and the answers may all seem to be correct. You have to figure out which is the best answer, and stay within the time limit.
Overall, at least in my opinion, the NET is not that difficult. However, I recommend getting the NET Study Guide from Eriworld.com, Ebay, etc.