I just finished a c++17 course. Where do I start
Just dive into the beginner section to test up your skills that you have acquired and have some experience of competitive programming questions, because only C++ knowledge won’t help you, the main thing that will help you out is techniques which we usually call algorithms. And that comes with practice, so just dive into beginner section of codechef practice.
Ya, I would also suggest you to do the Beginner - " https://www.codechef.com/problems/school/ " level questions at Codechef.
And as you are new to the language I think you should also do the questions from C++ section of HackeRank - “https://www.hackerrank.com/domains/cpp?badge_type=cpp”. By its end you will be pretty good at the basics of C++ practically .