Issue with Code- Help Requested - How do i take input from the user for this code ?

Based on user input i  need to show a message . Cant figure it out. Please help .

public class mrhqProjectClass {
    public static  void projectDingDong(integer inp){
        List<Integer> input = new List<Integer>();
        integer varNumber 	= 	12;
        input.add(varNumber);
        integer varNumber1	=	20;
        input.add(varNumber1);
        integer varNumber2	=	30;
        input.add(varNumber2);
        integer varNumber3	=	17;
        input.add(varNumber3);
        for(integer inp:input ){
            if(Math.mod(inp,3) == 0){
                string vardisplay ='Ding';
                system.debug(varDisplay);
            }
            else if(Math.mod(inp, 5)==0){ 
                string vardisplay ='Dong';
                system.debug(varDisplay);
                } 
            else if(Math.mod(inp, 3) == 0 && Math.mod(inp, 5)==0 ){
                string vardisplay ='DingDong';
                system.debug(varDisplay);
            }
            else if(Math.mod(inp, 3) != 0 && Math.mod(inp, 5)!=0){
                string vardisplay ='17';
                system.debug(varDisplay);              
            }
        }
    }
}