Verify Form data, then submit

When a user enters info into a Form, how would I verify the data and then submit the Form to another page that handles the data?

I need to have the form page open message boxes if the data the entered is incorrect and make the user try again, or if the data is correct, submit the form to another .asp page.

currently I just have this which just sends the form without checking the data:

<form name="form1" method="post" action="SendSuggestion.asp">

Thanks