Once you have created the validation functions for each of the fields on your form where the validation is required then you need to attach that validation to your form.
All of the validations will need to be performed immediately prior to submitting the form (and aborting the submission of the form if anything is found that is not valid). All cross field validations should also be performed at this point after the individual fields have been validated and before the form is submitted.
With text fields, text areas, password fields, and drop down lists you may also wish to call the validation for the individual field as soon as the person filling out the form moves on to another field so as to get them to correct any fields that they enter immediately rather than waiting until they have filled out the whole form before telling them that they have made an error in the first field.
The form field validation tutorials on this site will take you step by step through the various different validations that you may want to use with your forms allowing you to validate your visitor's input before they submit the form.
