Where can you get advice about wiring up speakers?

I've got a new life planned for a old analogue radio. I'm going to rip out its insides and replace with a Raspberry PI running RuneAudio. That bit, I can do. However, I'm at a complete noob with speakers. Conceptually, I understand the difference between passive and active, and I know that I'm meant to match impedance, but in practice... well that's a different thing.

Does anyone know of a good, step-by-step tutorial for wiring up a passive speaker to a Raspberry PI or some other similar device?
 
Yes, he can run the audio outputs into powered computer speakers.
That definitely doesn't qualify as having the ability to drive a speaker. The Pi will provide a line level signal, but not a speaker level one. The OP asked about wiring passive speakers, and DF96 was correct that he'll need an outside amplifier.

MrTP: I'm not very familiar with the Raspberry Pi, but I'm guessing that if it doesn't already have an 1/8" jack headphone/line output, then there's a shield that does. That's what provides your line level signal that you'll then need to plug in to a small amplifier module. (I'm new to this site and haven't looked around enough yet, but PartsExpress.com is my goto...) Something like this would make things easy, thanks to it even including a volume control knob:
Dayton Audio DTA-2 Class T Digital Audio Amplifier Module

If you're only installing one speaker, there are small mono amp modules available too. Just look around and do a little reading. It'll be pretty easy to figure out.

You also seem to be confused about impedance, so I'll just say it:
If the amp says "Min 8ohm", think of that as "AT LEAST 8ohm" and do not go less. Hooking a 6 or 4ohm speaker to it and cranking it up will fry the amp.
If you already knew that, forget I said it... :D