Also, your case statement must not have a space following the dollar-sign. It needs to be $choice. And if you are wanting this menu to repeat, you can put it in a while-statement like:
#!/bin/sh
choice=0
while [ $choice -ne 9 ]
do
echo "system monitor"
echo "
1) system paging
2) system file inf.
3) system disk inf.
9) exit
"
echo "select an option: \c"
read choice
case $choice in
1) sar -p;;
2) sar -u;;
3) diskusage -p;;
9) ;;
*) echo 'Invalid choice'
sleep 1;;
esac
done