Restricting information keyed into forms to a certain length

[hallm]

We're using an html form that posts to an asp page to add or update our database.  How can I provide a full proof method that the field lenghts are limited.  In other words when the user presses submit something that will check for length and not let the user key in more than a certain amount of text.  Also, how can you require that a field not be left empty, (Not be able to submit until a field be completed.)