You can add a piece of CSS to your form's custom code.
I'd try this:
<style type="text/css"> .wForm form { margin-top: 0; } </style>
To find out where this code goes, read: http://app.formassembly.com/pages/support/customization
If you've used the Theme Editor to design your theme, you can also alter the margin and padding in the 'Advanced Settings'. Let me know if you need more info on that.
If this doesn't help, please post a link to your page so we can see what's going on.