There are any number of designs out there for auto water change, hop onto some forums and search. www.monsterfishkeepers.com is a great site for DIY and automation projects. It really depends on how automatic you want it. Do you want it fully automated? If so then you are looking at a situation where you would want:
1) something to hold tap water and circulate it for about a day to make sure pH doesn't change much when you put it in the tank(may not be required if you have good tap water conditions or have it filtered in some way)
2) drip system that will drip house water into your holding tank. Typically done with an irrigation drip regulator/needle valve or some combination link that.
3) powerhead or pump which is engaged by a float valve that when the water level reaches certain height will turn the pump on and add the new water
4) overflow in a sump that will allow the extra water to drain to a house drain or another float valve system with pump
This of course assumes you have a sump system and all the necessary plumbing and household drain/water connections close at hand