How do you change electric heat to a gas heating system for the house? How much would this cost? How big of a job is it?

You basically replace the existing system with a new one. Depends on the age and type of old one as to how much and what needs to be changed

First of all is the elec heat your are replacing baseboard or is it a air handler with and electric strip? Is there nat gas service available to you or is this going to be oil? Do you have ductwork in the house? These are only a few of the questions to only get started. You also would let us here know if you are looking at Goodman(low end) or Trane(high end) system. There are many models with different SEER ratings available that also will change the price. Heaters are avail from 80% to 96% and air conditioning from 13-18%.

Its simple to say ,but not so simple to do,

1st, rip out the electric system

2nd, install gas system

And consider all the extras that are needed,

Yep, there is no way to convert an electric furnace to gas. You will need to remove the electric furnace, and install a gas furnace. You will need venting, electrical(changing from 240 to 120), gas supply and piping, and drainage(if going with a high efficiency furnace). Good news is, you should be able to reuse most of the ducting. Lets just say this is going to be well into the thousands. I suggest getting at least three bids, and go with the one that you trust.

Electric up stairs is base board. 1st level is forced hot air with heat vents. I have a burner in basement for 1st level. Can I make as one. How much would this coast?

electric up stairs is base board. 1st level is forced hot air with heat vents. I have a burner in basement for 1st level. Can i make as one. How much would this coast? Who do I call for this type of job and an estemate?

Who do i call for this type of job and an estemate?

HVAV contractor. Realize that it is unlikely a unit designed for 1 floor will be able to heat and cool 2 floors. In addition, you will need new ductwork for the second level which may necessitate tearing out the walls.