Ben
If you haven't already done so go watch Access Beginner Level 1  IT'S FREE

I don't see that you have any of Richard's courses AND I don't know where your experience level with Access is out side of this forum.  We can give you lots of links and help solve issues if you get stuck but jumping right in the middle with out learning the basics first you are going to just get frustrated.  Gotta learn to crawl before you can run.  
The questions you are asking are covered along the way "in general" if not specifically in the courses.

Here is one for instance that starts addressing Tax:

Access Expert Level 8

Tax Calculation is covered in Microsoft Access Expert 8

As far as a form to put a business and cashier. Do you mean to link to another table of information? Or just a form by itself?

For a form by itself, you make a form based on the table your using. STRONGLY recommend you go watch the entire beginners series if this is where you are.  For a form to link to another table, look at one to many relationships. https://599cd.com/blog/display-article.asp?ID=1724 and you can just make it a subform on your customer form

Again with the reports, if you just want to make your first report, watch the beginners series. If you want to get this new table added into your existing one, use a query to combine the tables.

A global "Settings" table is covered in Access Advanced 4