If your voltage measurement is valid at 1.2 volts, then your essentially new battery is essentially dead.
(I'm only questioning the validity of your measurement because if you are not familiar with voltage measurements and your meter is not auto-ranging, 12 volts can be mistaken for 1.2 volts. For the purpose of this answer, I'll assume that the reading is accurate.)
To take this any further on your own, you need to (1) fully charge the battery and (2) test the battery or have the battery tested.
The only rational way to fully charge this battery is with a battery charger, either in the vehicle or after removing the battery from the vehicle. If you don't have a battery charger and don't want to buy one, you can take the battery to a garage and have it charged for a nominal fee.
If you have your own charger and it's a "smart charger", the battery is so far discharged that smart chargers usually will not detect the battery and won't attempt to charge it. You can override this safety feature by following the charger's instructions or by momentarily connecting a portable jump starter to the battery while the charger is connected.
It can take 12-24 hours to fully charge a completely dead battery. If the battery will not charge, then you need a new battery.
Assuming that you were able to completely charge the battery, it must be tested using the proper equipment which puts a load of hundreds of amps on the battery for a few seconds and watches how the voltage drops. Good vs bad batteries have different voltage drop curves when placed under heavy load.
If you have a carbon pile tester, you can do this yourself. If not, most auto parts stores can test your battery at no charge in the hope that you will buy a new battery. Or you could ask the garage that charged your battery to test it.
If the battery has been fully charged and it tests "good", it should be re-installed and the car should start with no problems.
Why go through all this trouble?
When a car won't start, you don't begin by guessing what's wrong and replacing parts in the hope that you guessed right. You don't start a repair like this by replacing the ECM or the starter or the alternator. You diagnose. You test. You fix what you know is bad. No guessing. Then you test and diagnose some more if problems continue.
You have started that process now. You know that the battery is dead because you measured the voltage. Charge it and test it and replace it if necessary, and I guarantee that suddenly a bad ECM will no longer be on the list of possibilities, and you have saved hundreds of dollars in unnecessary repairs.
When your car is running with a known good battery, you can continue the diagnosis by starting a new question here. The question should start something like, "I have a (year, make, model) that had a totally dead battery. I have a known good battery installed now and the car is running fine. The battery voltage is (X) when the car is not running and (Y) when it is idling. Does this indicate a problem? How can I diagnose why the battery went dead?"
