Format the cell as Percentage by going to the paint brush icon, choosing the Format tab, then Percentage (you can tap the blue arrow for options).
If you want to know what percentage (more or less) this month's pay (C) is compared to the previous month (P), you would divide the current month by the previous month. C/P = %Change
P is the amount that you want to reference to. C is the amount that something changed.
For example (keeping it really simple). If you got paid $100 dollars last month and $50 dollars this month that would be:
50/100 = .5 = 50% You got paid 50% of the previous month's amount.
The actual result will be decimal. You could convert the result to percentage by multiplying the results by 100, but using the cell formatting does two things for you.
One: It formats the result as a percentage.
Two: It leaves the actually value as decimal, so you can use it in further calculations without having to divide by 100 first.