I need to add up two numbers input by the user. To do that, I create two input fields, retrieve values from them , using .val(), in two separate variables and then add them.
Use parseInt to convert a string into a number:
var a = '2';
var b = '3';
var sum = parseInt(a,10) + parseInt(b,10);
console.log(sum); /* 5 */
Keep in mind that parseInt(str, rad) will only work if str actually contains a number of base rad, so if you want to allow other bases you'll need to check them manually. Also note that you'll need to use parseFloat if you want more than integers.