How to accept input in dollar amount format?

how do we format a field so it is dollar amount format?  

For that I would suggest taking a look at the following widget: Fixed Format

It will allow you to predefine your format and to accept it as such on your forms.

You can see here how to add the widget to your form: How to Add a Widget to your Form, but do of course let us know if there is anything that we can assist you with and we would be happy to do so :)