First of all, work on writing titles that might help people to see your post. (See MensaWater's post after this one, seriously.)
if [ -f $1 ]; then
cat $1 | gpg --encrypt
echo "error: Could not read input file. Does it really exist? Is it a directory?"
echo "Usage: $0 <file>"
echo "file: the file you wish to encrypt"
But this should've taken maybe ten minutes of actually learning things to figure out, even if you've never used bash.