Validation of web-to-lead form generated in Salesforce

Hi
We have a wordpress site working well with forms created in Salesforce to take advantage of the Web-To-Lead function, all this works fine and enquiries are stored in our Salesfroce ORG.
The problem is there is no way of validating various form fields i.e. some times the form is submitted with no entries and also sometimes vital information is missing i.e. they have not given an email address.
Im not looking for anything fancy just a way of stopping forms being sent with incomplete information. So in a nut shell i need First name, Email, Company name validating.

Iv included the code below but have taken out our Salesforce ID.

If anyone can help id be most grateful.

Link to the page https://www.cnet-training.com/contact-us

<form action=”https://www.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8&#8243; method=”POST”><input type=”hidden” name=”oid” value=”SALE-FORCE ID REMOVED” /><input type=”hidden” name=”retURL” value=”https://www.cnet-training.com/form-submission-success&#8221; />
<label for=”first_name”>First Name</label><input id=”first_name” type=”text” name=”first_name” size=”20″ maxlength=”40″ />
<label for=”last_name”>Last Name</label><input id=”last_name” type=”text” name=”last_name” size=”20″ maxlength=”80″ />
<label for=”email”>Email Address:</label><input id=”email” type=”text” name=”email” size=”20″ maxlength=”80″ />
<label for=”phone”>Phone Number:</label><input id=”phone” type=”text” name=”phone” size=”20″ maxlength=”40″ />
<label for=”company”>Company:</label><input id=”company” type=”text” name=”company” size=”20″ maxlength=”40″ />
<label for=”title”>Job Title:</label><input id=”title” type=”text” name=”title” size=”20″ maxlength=”40″ />
<label for=”city”>City</label><input id=”city” type=”text” name=”city” size=”20″ maxlength=”40″ />
<label for=”country”>Country</label><input id=”country” type=”text” name=”country” size=”20″ maxlength=”40″ />
<label for=”course_interest”>Select the course(s) you are interested in:</label></p>
<select id=”00ND0000004q2O9″ multiple=”multiple” name=”00ND0000004q2O9″ title=”Course Interest”>
<option value=”CDCDP”>CDCDP?</option>
<option value=”CDCMP”>CDCMP?</option>
<option value=”CDCEP”>CDCEP?</option>
<option value=”CDCTP”>CDCTP?</option>
<option value=”CTPM”>CTPM?</option>
<option value=”EU CoC on Data Centre Efficiency”>EU CoC on Data Centre Efficiency</option>
<option value=”Data Centre Energy Practitioner DCEP”>Data Centre Energy Practitioner DCEP</option>
<option value=”CNCI Adv”>CNCI Adv?</option>
<option value=”Datacommunications Cabling”>Datacommunications Cabling</option>
<option value=”Fibre Optics in Int & Ext Environments”>Fibre Optics in Int & Ext Environments</option>
<option value=”Practical LAN Design & Implementation”>Practical LAN Design & Implementation</option>
<option value=”BICSI RCDD”>BICSI RCDD</option>
<option value=”Excel System Installation”>Excel System Installation</option>
<option value=”Draka System Installation”>Draka System Installation</option>
<option value=”Panduit Certified Installer”>Panduit Certified Installer</option>
<option value=”Fluke Certified Cabling Test Technician”>Fluke Certified Cabling Test Technician</option>
</select>
<p><label for=”message”>Message:</label><textarea id=”00ND0000004oVyD” name=”00ND0000004oVyD”></textarea>
<input type=”submit” name=”submit” /></form>