How to display a message with IF Statements?

I'm making an order form for a basketball store and I want a message that displays "The amount exceeds $1,000, do not process the order." for grand totals (Cell D20) over $1000.00. What I'm trying to do with that statement is that my basketball store will not process an order if it's $1000 or more. The order needs to be less than $1000. Somebody told me I should use these functions to get this to look right:

AND
IF
NOT
OR

How should I do this?

I want this to go into cell D23.

Note: If their are any other functions other than the ones I listed above, please feel free to share some other functions.