It's too bad your previous owner didn't maintain an equipment handbook. Most boat owners will keep the manuals in a folder somewhere.
The Autohelm is a UK company acquired by raymarine, they based their ST series of tillerpilots on the AH1000, but updated the tech features pretty quickly. .. so your unit will not go beyond basic functions. That's not a bad thing...just know its limitations. I still have the ah1000 unit that came with my boat when I purchased it in 1999. It's there for backup if needed.
Anyway, the link that Russ posted will give you a good idea of how the unit is mounted to the boat. Three connections... a mounting socket that holds the device to th boat, a tiller pin that connects the TP's pushrod to the tiller, a power/data connection via a four prong tiller pilot socket.
So now, take the ah1000 into the cockpit and start looking for these connections..... a metal, round head pin on the tiller; a flush mounted socket 90 degrees to starboard of the tiller pin; and a capped, weather proof tiller pilot socket. If you find every thing on deck, all that's left is to locate the power switch on the 12 volt panel. If you're lucky it'll be labeled, otherwise cycle through the panel till you see unit wake up.
Not much to do until you're out on the water. With the unit in the mounting socket you can manually extend or retract the pushrod with the plus /minus keys.....use this method to mov the pushrod so it can clip to the tiller pin. Unless the tp is actively steering, keep disconnected in standby. Once out in an area you feel comfortable using it, set a course with sails trimmed, then activate the steering function and the tillerpilot's computer and compass will begin its analysis and very quickly start making small corrections to keep the boat on course.
Finally you can find the ah1000 user manual at raymarine.com owner manuals in the "retired products " slot.