Answer 1

There were many errors in code ,now you can compare the following code to see what you did wrong

window.onload=function(){
document.getElementById("btn").addEventListener("click", function(){
var box=document.getElementsByClassName("box");

if(box[0].style.display=="none")
{
box[0].style.display="block";
}

else
{
box[0].style.display="none";
}
});
}

Answer 2

Would help a bit if you provided a link to your code...