what would be the best opening and the best defense for the 300 rating?


Below 1000 rating, I think it is actually more useful to learn to implement "opening principles" instead of memorizing a specific opening. https://www.chess.com/blog/KeSetoKaiba/opening-principles-again
Of course, there is nothing stopping you from doing both. You can use opening principles AND study your opening of choice. I made a video recently on how to choose your chess opening. I recommend taking that chess personality quiz I mentioned (and linked) in the video and then seeing what openings it recommends to you. Once you have the name of those openings, then you can research and study into it a little and eventually start testing it in your own games to see if you like it.