JavaScript Validation Needed for a Form

I am not a professional programmer and try to do some light coding. I created the form below doing some research in the web but I can't prepare JavaScript validation scripts so that user can't keep any field blank. I didn't even declare any ID for the form. Please come up with something so that I can proceed.

<form action="upload_results.php" enctype="multipart/form-data" method="post">
<table align="center" bgcolor="#CCCCCC" width="250px">
		<tr>
			<td align="left"><b>Exam Name:</b></td>
		</tr>
		<tr>
			<td align="left">
				<select name="ex_id">
				<?php
				foreach($exam_ary as $key=>$value) {
					echo "<option value='$key'>$value</option>";
				}
				?>
				</select>
			</td>
		</tr>
		<tr>
			<td align="left"><b>Date of Exam:</b></td>
		</tr>
		<tr>
			<td align="left">
				<?php
					  $myCalendar = new tc_calendar("date5", true, false);
					  $myCalendar->setIcon("calendar/images/iconCalendar.gif");
					  //$myCalendar->setDate(date('d'), date('m'), date('Y'));
					  $myCalendar->setPath("calendar/");
					  $myCalendar->setYearInterval(2000, 2015);
					  $myCalendar->dateAllow('2008-05-13', '2014-12-31');
					  $myCalendar->setDateFormat('j F Y');
					  //$myCalendar->setHeight(350);
					  //$myCalendar->autoSubmit(true, "form1");
					  $myCalendar->setAlignment('left', 'bottom');
					  $myCalendar->writeScript();
					  ?>
			</td>
		</tr>
		<tr>
			<td align="left"><b>Result File:</b></td>
		</tr>
		<tr>
			<td align="left">
				<input type="file" name="result_file">
			</td>
		</tr>
		<tr>
			<td colspan="2" align="justify">
				[You must upload either a <b>.xls (Microsoft Excel 2003)</b> or a <b>.pdf (Portable Document Format)</b> file]
			</td>
		</tr>
		<tr>
			<td colspan="2" align="center">
				<input type="submit" name="add" value="Upload">
			</td>
		</tr>
</table>
</form>

Code (markup):

 

Use javascript. You can find the sample code over the net. Sorry i forgot the link where i got mine.

 

You can try something like

function checkEmptyInput()  {
    inputArray = document.getElementsByTagName("input");
    for (var index = 0; index < inputArray.length; index++) {
        if (inputArray[index].type = 'text') {
            if (inputArray[index].value == "") {
                    alert( "Please fill all the fields." );
                    inputArray[index].focus;
                    return false ;
            }
       }
    }
}

Code (markup):

This will check if all the text fields are non-empty, otherwise throws an alert message.