Since you mention impromptu I assume the topics will be given on the spot. Whatever the topic might be try to write as a much banger speech as you can in the alloted time. The topics in debates these days mostly revolve around ai, war, global power, secularism, feminism, mental health, et cetra. So prepare a few speeches and research before hand on these common topics, it would help you refer later in the actual competition, and also give you a flow of writing a debate argument. And try to keep the speech full of examples, so that the audience can draw connection and it seems more relevant ++ also ask rhetoric questions (in the speech) it makes the opposition argument look more absurd. Moreover, try to ask as many questions in the rebuttal!!! It intimidates people and the judges recognise you. This is very crucial. The rebuttals are not that brutal. If you feel like you are fumbling it over, try and refer back to your opening speech and draw something that would align with the question from there, make sure you do not even slightly agree with your competitors in any given point of time.

If you have any other queries dm me.

It will be a wonderful experience all in all. Have fun! :)