What are main ideas I can write about to answer the question does anaesthesia have an effect on long term memory?

Anes & Memory. This is an emerging area of concern in my field after many years of feeling reassured that there was little risk. I would suggest that you focus on the two population segments that have been studied in more detail lately - the very young and the old & very old. I would also suggest addressing the effects on cognitive function in your paper as well.
Write about ? If you are doing a report on anesthesia and memory problems my starting point would be to read about the affects of cerebral hypoxemia and low blood pressure on cognition and memory. When one undergoes general anesthesia for major surgery sometimes the blood pressure drops to the point that the brain in not sufficiently perfused. This is uncommon. Then read about the various agents themselves.
Not conclusive. There are studies to show that cognitive impairment may follow general anesthesia in the elderly. That is why I do regional anesthesia if I possibly can avoid general anesthesia. Keeping blood pressure within a safe range and avoid hypoxemia is also important.
Types of anesthesia. There are many types of anesthesia, you could talk about that first (general, local, regional, topical). Then there are many kinds of anesthetics. List the classes of anesthetics and then you can check the sites about them and see under the side effects whether they affect memory and for how long. Good luck with your paper!