I planning to take CELTA course and just wondering if anyone knows about the lesson planning:
do we have to decide our own topic for TP?
No. You will have your own teaching group of about 6 teachers, with 3 teaching one day and 3 teaching the next. You will also have a text book that you will derive your lessons from, and it has to be done at least roughly in order, because you have real students and you want to actually teach them in a structured way. The other requirement is that you teach all of the different lesson types (main aims) by the end of the CELTA (vocab, listening, speaking, reading, functional language, pronunciation, etc), so which part of the unit you teach might be determined by what skills you've taught so far. So the book gives you the topic, but beyond that, it's pretty much up to you how little or much you use the activities in the book and how you want to adapt them.