The <script> tags are HTML -- they tell the browser that what follows will be scripting code, not HTML.
To comment out a line in JS you would use the double slash //. For multiple line comments you can use the slash-star pair as such: /* here is my comment */
var str1, str2, str3; //declaration of 3 variables named st
str1="<b>Hello World</b>"; //assigning a "string" to the variable
str2="<b>Goodbye World</b>";//assigning a "string" to the variable
str3="<b>The End</b>"; //assigning a "string" to the variable
Text within the main <body> tag will display AFTER any scripts above it in the <head>
var str4 = str1; //create a new variable, assign th value of str
str1 = "This is " + str3; //change the value of str1
str3 = str2; //assigning the value of str2 to str3
str2 = str4; //assigning the value of str4 to str2
Try removing quotes, variables and semi-colons. What kind of error messages do you receive?
Things to remember are include the <br> tag in your string if you want to break to the the next line. And of course, check your typing carefully!