A new PO file is created automatically with the
msginit command that will make a copy of the PO template available in the .pot file with your email address and other information.
When the template is missing you can open a bug on the project or send an email to the maintainer to let him know about the problem so he can add it. As a temporary solution you can run the command
make -C po update-po in the source directory of the project that will generate the template (you need to have run the autogen.sh/configure script which builds the Makefiles).
If your current locale is not the one in which you want to translate to you have to pass the argument
--locale=<xx> to the msginit command where <xx> is the language code.
Other programs specialized for PO files like Poedit have the ability to generate an initial PO file from the template, in case you use one of them, refer to their documentation for exact information in case you don't want to use msginit.