Where is the diagnostic port on my 1988 Subaru GL wagon with the ea82? Is it a 9 pin plug, or is it the 16 plug? Any help is appreciated.

There isn't a standard one.  You read the codes on the LED on the ECU, under the steering wheel.  There is a plastic panel that needs to be removed.

How do you know what code it is, and where and how does it tell me which codes are on it?

There is probably a link / thread on here with a list.  The factory service manuals have a table.
 

With the car idling, look at the blinking LED.  Count the long blinks, and the short blinks.  Long are the 10's place, short are the 1's place.  So 3 long & 4 short means code 34, for example.

 

If there is more than 1 code, they will list out in sequence.  The sequence is repeated endlessly. 

Most often there is only 1 code.

 

If you reply with the number /s you get, someone will reply with what it is.

The car is struggling on idle. I’ll look it over

Crummy idle.

Check -

Plugs

wires

look for vacuum leaks