man bash contains some useful information on boolean operators for files.
Here's what you need, with some comments:
# command line parameters are accessed via $1, $2, etc.
if [[ -f $1 ]]; then # the -f returns true if the parameter is a file, false otherwise
echo "$1 is a file"
if [[ -d $1 ]]; then # the -d returns true if the parameter is a directory
echo "$1 is a directory"
Edit: I misunderstood the question; however, I'm leaving the answer as is since it is useful for what I assume is the ultimate purpose of your script. If you just need to distinguish between files and directories, using -f and -d are a better way to go about it than looking for '/'.