What do I do if my furnace is blowing cold air

My first instinct is the tell you to call a plumber or HVAC guy.

Do you have any experience working on HVAC equipment?
What kind of systems is it? Gas, oil, electric, geothermal, etc,

Maybe try to post this in the HVAC forum? They can probably help you better. This is not really an electric issue.
Heating & Air Conditioning - Ask Me Help Desk

I'll echo Stan on this item.

It really comes down to what type unit you have.

For example, if you are using an Electric Forced Air unit with a heat pump, then you can expect some cool air in the system. If it is really cold, then the "Heat Pack" may be at fault or a single glow plug on the heat pack could be the problem.

This is best handled by the HVAC forum, I'll ask that this item be moved there.