My car's engine light is on and the car is shaking. What is wrong? on 2000 Mazda 626

This just started after I put some gas in it on yesterday. Steam is also coming from my car. I put water in it to make sure the heat gauge stays down. I also notice this: when I would turn the vent to heat I would not see the steam and when I would turn it to cold, I would see the steam. yep. If there be any old time mechanics out there, please give me your advice.

Asked by for the 2000 Mazda 626
Either bad gas, but if you are getting steam and running hot, you probably have a blown head gasket.
Hey thats what I have been told. Can you give me an estimate? How much would it take to buy a head gasket and to put one in?