So i'm thinking, that if I plug a speaker straight into the output of my guitar, it would work, but be almost silent. So how can I make it powered?

There must be some way of attatching a 9V battery to the speaker so it has actual amplification power?

Wiring diagrams please?
an amp is the thing that amplifies the signal. the speaker is just a speaker.

to add 'power' (to amplify the signal), you need to build an amp. check out the ruby or little gem, they're both pretty easy mini SS amps
The Little Gem is pretty simple to build. Here's a pic of the guts of a Little Gem I built for a travel amp. I put in a 4" speaker when I put it in an enclosure. That tiny one was just to test if the circuit had worked. And of course, I trimmed the perfboard.
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Do some reading as to how pick ups work, and how speakers work. Then find out what an amplifier is, what it does, and then how it works.