Depends. Cheapest is taking a fridge and converting it. If you are comfortable drilling holes in fridges, it is pretty easy. We got a cheap used fridge from an architectural recycling place, but any working fridge that can hold a 5# CO2 tank and a 5 gallon corny keg will work. CO2 tanks and corny kegs are kinda spendy. Basically you need a fridge, a keg, a CO2 tank, the faucet, the connectors, hoses. and a tap handle. Drill a hole in the door, put the faucet through, connect the CO2 to the keg and the keg to the faucet. Often times (at least around here) you can find some free/cheap stuff on Craigslist. I recently got 8 corny kegs for free (they were disgusting, but cleanable).

Expensive way is buy a commercial kegerator and then the CO2 and keg, but that is very easy. You might have folks selling cheap kegerators in your area too, so just check Craigslist and such (there are 4 in my area from $50 to $2500).

It might be expensive just for water, but for kegging beer it was worth it for us