Creating a text file in unix can be done through a text editor (vim, emacs, gedit, etc). But what you want might be something like this
echo "insert text here" > myfile.txt
That will put the text ‘insert text here’ into a file myfile.txt. To verify that this worked use the command ‘cat’.
cat myfile.txt
If you want to append to a file use this
echo "append this text" >> myfile.txt