>> Now, are you saying that I type in #include <string> literally, instead of replacing that "<string>" with a file or directory name?
Yes. Currently, in your Hello World.h header, you "use the <iostream> "dictionary" of commands". That's what you need to do with <string> as well.
Your understanding of headers and namespaces could probably use a refresher, perhaps find that section of your book/tutorial and re-read it. A #include basically copies the contents of a header file into that spot in the program. Standard headers are placed in <> brackets and do not end in .h even though they are all implemented as files. Your own headers are usually placed in "" quotes.
You almost never #include anything other than .h or .hpp files or files with no extension from the standard library. The .txt file you have included there looks suspicious, why are you #includ'ing it?
Normally, based on the code you are using, the top of your file would look like this:
using namespace std;