You're not alone; a lot of IELTS candidates have problems understanding the difference between 'No' or 'False' and 'Not given'.
One thing that I've found helps is to change the statements into questions; if you can answer the question 'Yes' according to the text, then it's true or yes; if you can answer the question 'No', then the answer is false or no. If you can't answer the question, then the answer is not given (unless you're looking in the wrong place).
Here are a couple of examples of T, F, NG questions from a text about hypnotism in the Official IELTS Practice Materials 2009.
37. Spiegel is more interested in scientific research than medical practice. (Is Spiegel more interested in scientific research than medical practice?)
"Speigel, however, is a clinician first and a scientist second. He believes the most important thing is that doctors recognise the power of hypnosis and start to use it."
Here, the text says that Speigal is more interested in medical practice - he's a clinician first - than scientific research, and so the answer is false.
38. Patients in the third group in Spiegel and Lang's experiment were easily hypnotised. (Were patients in the third group in Spiegal and Lang's experiment easily hypnotised?
"On average, Spiegel and Lang found the [third group] used less medication, experienced less pain and felt less anxiety than the other two groups."
Here, we cannot say whether patients in the third group were easily hypnotised or not; the text doesn't say that they were, and it doesn't say that they weren't, and so the answer is not given.
Hope that helps,
personal statement What I've said above applies to both Y, N, NG questions and T, F, NG questions.