To use PMT, position in the cell where the answer is to go. 
Select the function icon . 
Select Financial since this involves a financial transaction with a financial intuition, a bank.

Keep highlighting the abbreviations in the right hand column and noting the description of what each does on the bottom of the dialog box.

Select PMT and the box below appears.  If the grey box is covering the numbers, move it off them (by dragging the blue bar on top)

Click in each box for a description of what goes into the box which displayed on the bottom of the dialog box.

Rate is the interest rate per period.  If you know the annual rate then you will have to divide by the number of payments in a year (52 for weekly, 12 for month).  Remember the 2 cardinal rules of spreadsheet. 

1. Never type a value more than once.  ALWAYS point to a cell (usually the nearest) that contains the value.

2. Never do the math yourself.  ALWAYS instruct Excel on how to perform the calculation  (which cells, functions or math operators) to use.

So if the yearly interest  rate is on the sheet, point at it and divide by 12 for a monthly payment.

Nper is the number of payments.  If the number of years the loan is for is on the sheet, point at it and then multiply that by of payments in a year (52 for weekly, 12 for month).  Remember the 2 cardinal rules of spreadsheet. If a number or a factor of a number is on the sheet, point to that cell, do not retype it.

PV stands for Present Value.  How much is the loan today?

As you enter numbers and cell addresses in the boxes, the current value will appear to the right giving you the opportunity to see that you have entered the correct value or cell address.

There is NO need to enter Fv nor Type.

Press enter or click OK.  A negative number will appear since you are taking money out of your pocket.

Manually calculate to see if the number makes sense.  It should be more than the amount borrowed dividend by the number of payments since they is interest.   But it should be less than the amount borrowed put the annual interest (amount borrowed times the interest rate/12 times the number of years) since with each payment you are reducing the amount that you still owe the bank  and thus the amount of interest the back collects.