Don't.  "String*" doesn't exist, you cannot have a native pointer to a
garbage collected object.

To get a pointer to the characters inside the String:

String^ s;

interior_ptr<const wchar_t> p = PtrToStringChars(s);

(Note that .NET strings are immutable, you are not permitted to overwrite
them in place, also they are Unicode).

If you need to convert to ASCII (single byte character sequences), there's
Marshal::PtrToStringAnsi.  Also the String constructor accepts a character
pointer, so you can do

wchar_t* p = L"My native Unicode string";
String^ s = gcnew String(p);