There are many places where you can learn C++, both online or in class, such as AlphaStar Academy. We recommend the best way is to learn is to compete at USACO.
As USACO competitor ourselves, we also teach computer science for students who are interested in competing in USACO. Our focus is to teach advanced algorithm that enables students to compete effectively in USACO. Those 13 classes, from CS201 to CS504, are now available to public on Google classroom.
We also have a class that explains most recent USACO Platinum problems, and we have published it here on youtube