The first parameter passed to the script is $1, the second $2 and so on
If you want to to check more then one possibilities of parameter passed to the script then a case is useful
case "$1" in
start)
# start
command 1
command 2
command 3
;;
stop)
# stop
command 1
command 2
command 3
;;
*)
# anything other
echo "Use: {start|stop}" >&2
exit 1
;;
esac