Bourne Shellscript

this is a bourne shellscript to  start an at command. can you please tell me how to make the program exit?
 on a few occasions i need to exit the script without it running throuhg any more code. there is a comment at one point (when the filename is invalid)

!/bin/sh

if [ $# -eq 2 ]
then

        if [ -f $1 ]
        then
                checksum=1
                optorarg=1
                echo parameter 1 has been verified

        else
                checksum=0
                echo file was not found or was a directory
                echo ------------------------------------------------------------------
                echo would you like to create a new file??
                echo 1 = yes
                echo 0 = no
                echo enter 1 or 0 depending on choice:
                read newfile

                if [ $newfile -eq 1 ]
                then
                        echo program will now create a new file for you
                        echo please remember to save afterwards!!
                        vi $1
                elif [ $newfile -eq 0 ]
                then
                        echo program will now output contents or current folder
                        echo -------------------------------------------------------------------
                        ls -l
                        echo -------------------------------------------------------------------
                        echo please enter the real filename:
                        optorarg=0
                        read option1

                else
                        echo file will now assume you wanted to create a new file
                        optorarg=1
                        vi $option1
                fi
        fi


elif [ $# -eq 0 ]
then
        optorarg=0
        echo no parameter found.
        echo System will now output contents of current directory
        ls -l
        echo please enter parameter 1:
        read option1

                             if [ -f $option1 ]
                then
                        checksum=1
                        echo Thankyou, parameter 1 verified
                else
                        echo Error!: filename invalid!!
                                #how can i exit script here?
                        checksum=0
                fi

        echo Please enter Parameter 2:
        read option2
        echo thankyou...
        checksum=1

else
        echo you have entered one or more than 2 parameters. program will now exit
        checksum=0
fi

if [ $checksum -eq 1 ]
then

        if [ $optorarg -eq 1]
        then
                at -f $1 $2
        else
                at -f $option1 $option2
        fi

else
        echo goodbye
fi



Sean