My experience with adding a remote starter a 2001 Pontiac Montana minivan which I assume is typical:

There has to be a key with a "chip" in the ignition before it will start.

I had to get a module that gets hidden under the dash with a wire loop extendsd to and wrapped around the ignition switch.

An extra working key with a chip goes inside the module and it transmits the signal from the chip to the ignition system as if a key was in the ignition switch.

This means you do not have a spare key anymore but you can buy them inexpensively on EBay and elsewhere like $15 or less vs $75 at a dealer. You can get them cut to match you existing key at Home depot for a buck.

Then you have to follow a routine found either online or in your owners manual to program the chip in the new key using your original working key.

The chip is a programmable transponder that receives a signal and then transmits a signal back with a code which is unique to your car. This programming can be done using an existing working key, Sometimes you need to insert BOTH working keys before you can program the third one.

I hope I haven't repeated anything already posted.