You need to seek to the end of the file before writing to it, see
http://www.cplusplus.com/reference/ostream/ostream/seekp/[
^]
So your call would look something like:
if (myfile.is_open())
{
myfile.seekp(0, ios_base::end); cout << "Enter the title of the new show: ";
[Edit]
Alternatively, you can do this when you open the file with the following overload:
std::ofstream myfile ("shows.txt", std::ofstream::app);