I was given a task to create a form, which I have done

Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Billing Form</title>
<script type="text/javascript" src="jsAssignTwo.js">
function formInfo()
{
}
</script>
<!--CSS for formatting the form-->
<style type="text/css">
    .container {
        width: 200px;
        clear: both;
    }
    .container input {
        width: 100%;
        clear: both;
    }

    </style>
</head>
  
<!--div for formatting purpose-->  
<div class="container">
<body onLoad="setFocus()" bgcolor="grey">
<form action="http://cswin2k5/echo/default.asp" method="post" name="billingForm">
<h2 id = "welcome">Billing Information</h2>
Title: </div>
<input type="radio" name="Titles" id="titleOne" value="Title" /> Mr.
<input type="radio" name="Titles" id="titleTwo" value="Title" /> Mrs.
<input type="radio" name="Titles" id="titleThree" value="Title" /> Miss.
<input type="radio" name="Titles" id="titleFour" value="Title" /> Ms.
<div class="container">
<p>First Name*:
<input type="textbox" name="first" id="first" onBlur="this.value=trim(this.value)" onkeyup='capitalize(this)'></p>
<p>Last Name:
<input type="textbox" name="last" id="last" onBlur="this.value=trim(this.value)" onkeyup='capitalize(this)'></p>
<p>Address:
<input type="textbox" name="address" id="address" onBlur="this.value=trim(this.value)" onkeyup='capitalize(this)'></p>
<p>City*:
<input type="textbox" name="city" id="city" onBlur="this.value=trim(this.value)" onkeyup='capitalize(this)'></p>
<p>Province:
<input type="textbox" name="province" id="province" onBlur="this.value=trim(this.value)" onkeyup='capitalize(this)'></p>
<p>Postal Code*:
<input type="textbox" name="postal" id="postal" onBlur="this.value=trim(this.value)" onChange="this.value=this.value.toUpperCase()"></p>
<p>Country:
<input type="textbox" name="country" id="country" onBlur="this.value=trim(this.value)" onkeyup='capitalize(this)'></p>
<p>Phone*:
<input type="textbox" name="phone" id="phone" onBlur="this.value=trim(this.value)"></p>
<p>Email*:
<input type="textbox" name="email" id="email" onBlur="this.value=trim(this.value)" placeholder="useremail@domain.com"></p>
<p>Credit Card Holder:
<input type="textbox" name="holder" id="holder" onBlur="this.value=trim(this.value)" onkeyup='capitalize(this)'></p>
<p>Credit Card:</div>
<input type="checkbox" name="visa" id="visa"> Visa
<input type="checkbox" name="master" id="master"> MasterCard
<input type="checkbox" name="american" id="american"> American Express</p>
<div class="container">
<p>Credit Card Number:
<input type="textbox" name="credit" id="credit" onBlur="this.value=trim(this.value)"></p>
<p>Expiration Date:
</div>
<select name="Exp month" id="expMonth">
<option value="blank" selected></option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="8">8</option>
<option value="9">9</option>
<option value="10">10</option>
<option value="11">11</option>
<option value="12">12</option>
</select>
				
<select name="Exp year" id="expYear">
<option value="blank" selected></option>
<option value="2014">2014</option>
<option value="2015">2015</option>
<option value="2016">2016</option>
<option value="2017">2017</option>
</select>

<p><input type="button" value="Proccess Payment" onClick="formInfo();"/></p>
</form>


</body>

</html>
I then made a .js file and did all the validation, which seemed to work perfectly fine. However I was told that each functions of mine needed to be independent from my HTML code and should be able to pass the value to the function in .js and get a return value (with no getElementById in the .js file).

.js file
Code:
	// Global variables used for form
    var missing = false;													// Boolean error message check
    var error = "";															// Error message

// Set focus on the first field 
function setFocus(f)
{
	// Set focus on first name
	var f = first.value.focus;
}
	
function firstName(fN)
{
	// Validate the first name
	if (document.billingForm.first.value == "")
	{
		missing = true;
		error += "\nPlease provide your first name";
		first.focus();
		first.select();
    }
}
function cityValidate(cV)
{
	var cityLength = city.value.length;										// Checking length for city
	
	// Error check for City and has to be longer then 3 characters
	if (document.billingForm.city.value == "" ||  cityLength < 3) 
	{
		missing = true;
		error += "\nPlease provide your city must be greater then three characters";
		city.focus();
		city.select();
	}
}
function postalCode(pC)
{
	var rePost = /^[A-Z][0-9][A-Z]\s[0-9][A-Z][0-9]$/;						// Postal code expression
	
	// Postal code validation and format
	if (!rePost.test(document.billingForm.postal.value))
	{
		missing = true;
		error += "\nPlease provide your postal code. Ex: N1E 1E9";
		postal.focus();
		postal.select();
	}
}
function phoneNumber(pN)
{
	var rePhone = /^\(?(\d{3})\)?[\.\-\/\s]?(\d{3})[\.\-\/\s]?(\d{4})$/;  	// Phone number expression
	
	// Phone number validation and format
	if (!rePhone.test(document.billingForm.phone.value))
	{
		missing = true;
		error += "\nPlease provide your phone number Ex: 5192125598, 519 212 5598, or (519) 212 5598";
		phone.focus();
		phone.select();
	}
}
function emailAddress(eA)
{
	//Email validation with expression
    if (email.value.search(/^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/) == -1) 
    {
        error += "\nInvalid email Ex: usersemail@domain.com";
		email.focus();
		email.select();
    }
}
function errorMessages(eM)
{
	// Show all message in one alert box
    if (missing)
    {
	// calling all messages
        alert("You have enter the following data incorrectly: \n " + error);
    }
    else
    {
		// if form filled out correct inform customer
        document.write("Thank you for submitting your information");
    }
}
// Capital on every letter function
function capitalize(capital)
{
    capital.value = capital.value.charAt(0).toUpperCase() + capital.value.slice(1);
}
// Trim all white spaces
function trim(trimSpace)
{
	return trimSpace.trim();
}