You should set your output impedance to match the load (I assume you have a valve amp).
I would advise trying to use the highest impedance tap on your output transformer as this will utilize the most number of secondary windings, which is theoretically the best way of using the transformer.
Assuming you have a 2x12 cab you can wire the speakers in series (add the speaker impedances together), or in parallel (half the impedances assuming they are equal), however there are differences in how speakers in parallel and series behave. I have posted on this elsewhere, but in a nut shell parallel wired speakers have better damping and hence a tighter bass response.
For a 2 x12 cab I would advise using two 16 ohm speakers in parallel to give 8 ohms. This will give yopu the option of running another 8 ohm cab with it (4 ohm in total). If you use 2 x 8 ohm speakers in parallel you would have to match this to 4 ohms and then you couldn't use another cab.
If you have a 4 x 12 cab then these are usually wired series/parallel. 4 x 16 ohm speakers will give a 16 ohm cab.
If you are wiring this up yourself it is a good idea to check the DC resistance with a multimeter (a £5 Maplin job is fine) before you run an amp into the can. Mismatching the load in a valve amp can cause the amp to be damaged. The Dc resistance of a speaker is a few ohms lower than the impedance. I would expect an 8 ohm cab to measure 6—7 ohms, and a 16 ohm cab to measure 12-14 ohms.
