Using Each function

Using Each function

I am new to Jquery.I am working on Dynamic validation using jqueries.

My code is:
<form class="form" id="commentForm" method="get" action="">
 <div class="cmxform" >
    <input type = "text"  size="25"/>  
    <input  type = "text"  size="25"/>  
    <input  type = "text" size="25"  />
    <input  type = "text" size="25"  />
    <input  type = "text" size="25"  />
    <input class="submit" type="submit" value="Submit"/>
<div>
 </form>
all fields are email fields.
1.In the above code,  I don't know How to use each function
I have to check the following conditions
1.clicking the submit button,It should check the first column have the value(null or not null),If null it throws the alert popup.Other input text not need.
2.Otherwise I am entering the value in third input box it also check first and Third input box.

Is it possible in Jquery.Please some one help to get my answer.