My 2005 Pontiac Bonneville is overheating.I can't even find the valve on the radiator.. seems like the coolant is all there.water pump? Thermostat? Where do I begin?

okay....there are a lot of factors here---the fact that you cannot find the petcock is normally front and center, but that's beside the point...when doing a coolant flush (sounds like you may have to on account of having tap water in that) you'll simply remove the lower hose and all the coolant (well most of it) should come out and be captured into five gallon buckets for recycling at the auto parts store...yes, even if it's rusty....once having rid the system of rusty coolant, change the thermostat...you'll find this one at the end of the top hose under the "water neck" get a 195 T-stat replacement~...next give the waterpump a jiggle and inspect the bottom shaft input...there's a "weep" hole that will let coolant out and drip and eventually will cause overheating....Hope to god's someone hasn't put HOSE water in a panic after overheating such that the block will be cracked and the coolant will RUST everything in it's path immediately and must be changed with 50/50 ethylene glycol and distilled water mix~ ...there's a start...once filled up we will check the efficiency of the radiator and it's ability to put the fans on when boiling temperature is reached~ a spot thermal gun is handy to find the hot spots~