I have a form that I need some validation before

HTML Code:
<form name='leadform' id='leadform' action="http://www.something.com/lander/formmail.php" method='post' class="Search" class="validate">
    <input type="hidden" name="env_report" value="REMOTE_HOST,REMOTE_ADDR,HTTP_USER_AGENT,AUTH_TYPE,REMOTE_USER" />

    <!-- STEP 2: Put your email address in the 'recipients' value.
                 Note that you also have to allow this email address in the
                 $TARGET_EMAIL setting within formmail.php!
    -->
    <input type="hidden" name="recipients" value="something@something.com" />

    <!-- STEP 3: Specify required fields in the 'required' value - or leave unchanged.
            NOTE: DO NOT put your email address and name here.
            "Your email address" and "Your name" are error messages for your users to see,
            not placeholders for you to replace.
    -->
    <input type="hidden" name="required" value="First_Name:Your first name,Last_Name:Your last name,Phone_Number:Your phone number,Email_Address:Your email address" />

    <!-- STEP 4: Put your subject line in the 'subject' value. -->
    <input type="hidden" name="subject" value="Canada Lead Landing Page Lead" />

    <!-- ALL DONE! Visit our site for HOW TO guides and more complex features. -->

    <!-- this derives (creates) "email" and "realname" special fields from the input fields -->
    <input type="hidden" name="derive_fields" value="email=Email_Address,realname=First_Name" />

    <!-- this excludes the "email" and "realname" special fields from the body of the email you receive -->
    <input type="hidden" name="mail_options" value="Exclude=email;realname" />
                        	<div class="row">  
                                <label for="First Name" class="label">First Name:</label>
                                <input name="First Name" id="First Name" class="input" class="required" type="text">
                            </div>
                            <div class="row">  
                                <label for="Last Name" class="label">Last Name:</label>
                                <input name="Last Name" id="Last Name" class="input" class="required" type="text"> 
                            </div>
                            <div class="row">  
                                <label for="Phone Number" class="label">Phone Number:</label>
                                <input name="Phone Number" id="Phone Number" class="input" class="required" type="text">             
                            </div>
                            <div class="row">  
                                <label for="Email Address" class="label">Email Address:</label>
                                <input name="Email Address" id="Email Address" class="input" class="v2_email" type="text">       
                            </div>
                            <div class="row gap">         
                                <label for="Make" class="label">Make / Model of Vehicle:</label>
                                <input name="Vehicle Interested In:" id="Make" class="input" class="required" type="text">
                            </div>     
                            <div class="row gap2"> 
                                <input name="field" id="financing" value="" class="radio" type="checkbox">
                                <label for="financing" class="label label2">I need financing!</label> 
                                <input name="field" id="Trade-in" value="" class="radio" type="checkbox">
                                <label for="Trade-in" class="label label2">I have a Trade-in!</label> 
                                <input name="field" id="shipping" value="" class="radio" type="checkbox">
                                <label for="shipping" class="label label2">How much for shipping?</label> 
                            </div>    
                            <div class="row">
                            	<input type="submit" value="Find My Car" class="button" type="button">