2002 Malibu Heating Issue

Hi,
 
I'm new and this looks like "the" place to be for my question.  I've got a 2002 Malibu sedan, and as of last Friday, the heater is now going amok.  Upon starting the car, it appears to get warmer but within ten minutes the heat stops and it's just cold air blowing.  5-10 minutes after that and it starts to heat.
 
So, we're guessing it's a thermostat issue... ????  How easy is it to replace, or is this one of those "hard to reach" things that needs to be taken to a mechanic to use some special tool to use for five minutes of work and a $100 bill?
 
Thanks!

most vehicles its not hard, but if you dont know wht your doing just take it in, for someone who knows takes about 45 minutes to an hour......