Total noob needing starting point

I consider myself computer savvy and I can generally pick up on things real quick, but this JavaScript is giving me a headache.  I have Acrobat DC and have created a couple simple forms so far.  The simple calculations are easy enough, but now I must delve into the more complicated scripting calculations and I just don't understand where to start.  I've read all sorts of blogs and forums, studies the instructions provided by adobe and i seem more confused. 

In the example form shown below, I want to have an employee enter a few fields and have the form calculate the rest as specified below:

1. Column "Pay Date" PaymentD_1 should equal Repayment_Start_Date as entered by the employee.

2. The rest of the PaymentD_xx should be PaymentD_1 + 14 days so that it is every two weeks.

3. Total_Due needs to be Amount, plus sales tax if the Sales_Tax_Included box is unchecked, otherwise it equals Amount.

4. Column "Amount" would be Total_Due divided by 13.

5. Column "Balance" would be (obviously) Total_Due minus PaymentA_xx.

6. Finally, I want each Signature box to send the completed form to certain email(s). 

I am looking for direction, not a written script.  I want to learn how to write this so that I can continue in the future.  Any help you can provide will be greatly appreciated.  Thanks in advance.