I really want to do a PhD. However I have no clue to as to how to chose a topic. Do I choose something that interests me or do I choose something that is the need of the hour ? For example the need of the hour is related to technology or AI or environmental. However I have no interest in these. How do I chose a topic that satisfies both ??

Read up (this is called a literature survey) and then look for interesting problems to solve (or knowledge gaps) in the area of interest. Usually, good research papers conclude with a section called "limitations of current work". These will trigger new ideas. Remember, a Ph.D. is a degree you should ideally complete in 4 years. Don't have any romantic ideas that you are out to solve the world hunger problem. Choose a problem (and a supervisor) so that you can complete the work in the assigned time. Be clinical about it.