How do I file a Paypal dispute?

I have tried to file a dispute and it told me that the type of transaction was not eligible for a dispute. I know I must be doing something wrong.
First of all when I pay someone paypal what catagory should I click?
Also when you file your dispute it asks you to catagorize the transaction.
I am not sure here either...
Non-Receipt of goods OR Goods Not-as-Described

then it ask you to catagorize it AGAIN....I do not know this either.

Anyone with experience at this?
PLEASE!

I always do my payments as a service. And have filed 2 disputes, one in progress now, as non receipt of goods. Then send them a note that they did not complete their end of the service, and that I can no longer use them and to pay me back.
Hope this helps!