It may be a wrong approach, but without creating a plugin, one of my seniors, once did a WordPress Shopping Cart project by creating extra tables for her custom purposes. What she did is to follow the other existing tables in WordPress database.
Suppose what wp_posts table has by default is an ID. She opened up the structure of the table and see:
+=======+============+===========+============+======+=========+================+
| Field | Type | Collation | Attributes | Null | Default | Extra |
+=======+============+===========+============+======+=========+================+
| ID | bigint(20) | | UNSIGNED | No | None | AUTO_INCREMENT |
+-------+------------+-----------+------------+------+---------+----------------+
Then she followed a similar things to her own table's ID's structure. And so on... And where necessary she made a new field with her personal choices.
It's her process— stealing structures from built-in WordPress db.
This was her methodology to the new table in WordPress for custom purpose. You can simply follow it or any other method proposed by any pro.
But being a newbie you should have to practice much with SQL & PHP first, it'll help you to understand its basics. All the very best.