working on a c sell script

I think I understand the concept of it, which is:

filename >> file.txt (to appaend)

or filename | tee -a file.txt (to append)

The problem is that my shell script is used with several parameters, and these commands don't seem to work with just filename. They both get a blank screen where I can type whatever until I quit, with nohing saved to file.txt

All the tutorials I googled don't mention parameters, so I don't know what to do

help!
thanks!