How do I make a reference to a cell such that it stays unchanged when I do a fill down? I have for example a sales tax amount reference in my formula, and when I fill down the reference to my sales tax cell is incremented too - I want it to stay fixed!

What you are looking for is called an Absolute reference.

To make an absolute reference you add a dollar sign on each vector of the address.

So, for an absolute reference to cell A1 - you'd write $A$1

Here is an example:

123

34

45

56

lets write a function for cell D1

=$A$1+B1+C1

Now, we fill down column D and the next formulas will be (for D2,D3,D4):

=$A$1+B2+C2

=$A$1+B3+C3

=$A$1+B4+C4

So, you see, the absolute reference doesn't change while the other ones fill correctly.