Hi,
I am new to e-commerce!

I would like to know if it is a better practice to create a table (i.e. mysiteusers) that will hold the usernames and passwords of the users of the site or it is better to rely on the Users of the rdbms (mysql).

Users of the site might have different privileges (select, insert e.t.c) and there may be thousands of users. Encryption of some fields (i.e. password) might be necessary.

If it's ecommerce, you won't be giving your end users direct access to your database, so you need a different table.

"That depends". Some e-commerce sites need user tables to encourage repeat business, and others are one-off purchases that don't require them.

Understand your products, your target market, and their buying patterns. Then ask yourself that question.