It's worth picking up some basic gdb skills. Use of ddd can help with this. On windows you can use VS for most of this of course. Picking up the basics of valgrind will also help you.
Get comfortable with the preprocessor. Get comfortable with Makefiles. Get comfortable pulling in library headers and binaries as needed.
I was (mostly) a C programmer for over a decade, but that's about all I can think of right now!
And someone below has just triggered me - FFS use stdint.h!
You absolutely need to know in C++ too. Modern C++ just gives you tools to express ownership in code. Rust goes way further and gives you compile time correctness of your lifetime handling
Pointers, stacks (one in ever 23.7 bugs is a stack smashing bug), bit bashing and endianness, types and coercion at the byte level (see also: pointers, bit bashing), C strings, the stupid rules about when a variable's value is actually written to memory that need to die in a fire, memory allocation/clearing/copying/ownership/freeing, ALWAYS CHECK RETURN CODES, what the heck an lvalue is.
This book is fun: