Adding and Reading data to custom table

I am trying to add a custom table to the WP database and fill it with the user data. Queue is for moderation and post it as a post to wordpress on acceptance.

I have a template for single with a form requesting Data from browser. I am passing this form data to a php file to check and process which is under Template dir/files/process.php

I am able to e-mail the data collected. but I want to semi-automate the process. Thus I want to add the Data to a table in WP database. That’s where I am now. $wpdb returns error. try to add a function in function.php file. No luck!

Can anyone help me here?
tks