Send data using post to custom plugin file

Hi,
I’m writting a plugin to extend my wp site. The basic idea is to create a table in the database to store informations about about target audience.
The plugin creates a table on plug-in install.
Next I added a menu page wich has a form in wich the user can input the person name and some info. The problem is that I don’t know what form action to use. I tried action='/wp-admin/admin.php?page=plugin-opt' wich is the setting page for my plugin but when I var_dump($_POST) I only receive the submit button.
Please help me understanding how to receive input from back-end form and insert it into the db.

Thank you