Are you wanting to input just one file name or many? If just one the easiest approach would be to give it as a parameter to the script file and access it via the variable $1. So your comand line would be something like:
and then inside the script $1 will have the value of outputfilename.
If you need to enter a number of filenames one after the other, then the read function would be a better option. Again, fairly straightforward; in the script a line like:
would put the filename entered into the variable $filename. Have a look at this link
for more details, and for information about shell programming in general.