How do I read the first line of a file using cat?

You don't, use head instead.

head -n 1 file.txt

You could use cat file.txt | head -1, but it would probably be better to use head directly, as in head -1 file.txt.

There are many different ways:

sed -n 1p file
head -n 1 file
awk 'NR==1' file

You don't need cat.

head -1 file

will work fine.