how to make textbox should be mandatory to be filled up

[Bhutan123]

<tr>
td>Enter Viva Marks (20%):</td>
<td><input type="text" id="VivaMarks" name="VivaMarks"  maxlength="4" onKeyUp="numericFilter(this);" value=" <?php if(isset($esubjectID)){echo $eViva;} ?>" ></td>
</tr>

[Dormilich]

that depends a little bit on the used browsers. if the support HTML5 form, you could simply add the required attribute, otherwise you would need to write a JavaScript that checks before submitting that the text field is properly filled (and if not cancels the submit).