GMC Repair: Heat problem/Thermostat, coolant level, coolant leak

Question
Hi there, and let me thank you for your time right now.  I have a '99 GMC Yukon SLT, with a coolant leak (somewhere) which could be quickly fixed with more coolant this just started, and it is about 8 degrees out right now.  So as you can imagine I haven't spent much time out under the hood.  It just got alot worse.  As I was driving into town this morning the thermometer got REALLY high, infact it was buried.  So I headed home, and let it sit and cool down.  Once it had cooled off for about two hours, I went to check things out, the coolant level is where it should be, but when I started it the collection tank was bubbling like a water cooler.  At this point I also have the heat cranked, the temp is at 210, but cold air still blows, not warm, but as if the A/C is on.  So could it be the therostat?  I actually don't know where that is(I am mechcanically inclined though)Please help!  Thank you!  Also on a side note if you need ANY info related to CATV/Audio/Video/Computer Networks/ Really anything with a wire and/or button give me a shout I would be more than happy to help you.  

Answer
hello clay
thanks for the help i will sure let u know if i do
this could be a bad thermostiac . also it could be a pluged rad . neax time start it up and fell the top if hot should be feel botton if cold then it is pluged change it will make it boil pete