I'm pretty new beginner, please give me an example string "003C" to hex 0x003C
No. If you really have a string containing "003C", you should be able to google strtoul() to figure out that it takes three arguments (the string to convert, a place to store a pointer to the first non converted character, which can be NULL, and the base (16 for hex)), and that it returns one value - the result of the conversion as an unsigned long.
There is even an example of calling the function, but calling strtoul() is so trivial that an example is not needed.