I'd start with the Wikipedia article on Geothermal Heat Pumps which is pretty good at giving the overview.
It's a bit hard to do a 'small scale' system which you might expand later if you are laying pipe in the ground or in a pond. You probably don't want to do that twice. But an open system might give you a chance to play with the ideas. If you search around online there are a lot of DIY projects that can allow you to experiment with geothermal. Despite the the comments of others, it's not rocket surgery ;) You could build a small heat exchanger or put in a small open loop system in front of your existing HVAC and let the open loop geo system do most of the work and have the existing system only kick in if the temp gets out of bounds.
What IS hard is DIY a system that will be efficient and save you money. Experimenting and understanding how geotherm works is a great project where you can learn a lot. But it's hard to beat the efficiency of commercial systems.
It is a good idea to talk with local installers, but be careful of which ones you talk to. If they are not specialists in geothermal they may not have good info. Make sure they have experience designing and installing and then get info appropriate for your geography.