The TOEFL writing section includes an integrated task and an independent task, which are designed to test students' ability to communicate through writing in an academic environment. The writing section is the last section on the TOEFL.
Check out: TOEFL Writing Tips
If the TOEFL writing section is tough for you, then you had better watch this video in which Lucas from Magoosh points out the two most common mistakes students make.
Copying from the text
One mistake a lot of students make is they focus too much on the text in the integrated essay. The integrated essay has a text and then a lecture. You have to write about what you have read and heard. Some students spend too much time writing about the text, while actually the task is always to summarise the lecture. So if you spend too much time focusing on the reading that causes a problem. It becomes especially problematic if you use the same words that are in the text because that makes it look like you didn't understand the lecture. And if you didn't understand the lecture, then it can be assumed that you are going to have trouble understanding lectures at real English-language universities. You should ensure that you stay focused on the lecture and don't copy sentences from the text.
Check out: How to Reduce Test Anxiety in the TOEFL iBT
Lack of variety
The other mistake is the lack of variety. What does this mean? If you are reading an essay, and every sentence has the same basic structure, it looks like the writer doesn't know any other structures. What you need is a mix of different sentence types. One of the ways to do that is to focus on the length of your sentences because in doing so you will have to use different types of sentences.
Watch the video to hear a more detailed description of the mistakes and also tips on how to avoid them.