brand new to this

These days, you can get an amplifier on a circuit board for very cheap. I'd grab one of those.
There are lots of 2x15w amp boards that'll run on 12v DC - add a holder for 8x AA batteries or a 12v wall-wart power supply, and you're good to go.

You'll use one amp channel for your current speaker, and when you want to expand, there's a second amp channel waiting to be used so you can have stereo.

Chris