Using Javascript is as simple as:
a=5;
b=7;
c=a*b;
alert(c);
As you probably guessed, we used Javascript like a Calculator, the above program alerts "35".
Some more explanation:
a=5; // "a" is now a variable that holds the value 5
b=7; // "b" is now a variable that holds the value 7
c=a*b; // "c" is a new variable, it's calculated as 5x7 - it's equal to 35, it's the value it's holding now
alert(c); // "alert" is a function that makes the browser pop up whatever you put in there, annoyingly, you probably saw them before :)