Resetting the computer is a totally different thing from resetting the oil change reminder. One does not affect the other. You can use an Internet search engine to search for "reset 2003 Pontiac oil change reminder" and that should eventually get you the answer to that one. But the check engine light is a separate and more pressing issue.
Whenever the check engine light is on; it means the computer has identified a problem. What you describe sounds like the engine is now running in "limp in" mode; which happens when a problem prevents the computer from using the usual sensor inputs.
But even if you had a code scanner; it wouldn't fix the car. All it can do is to display the code numbers it found, and some of the more expensive models will also list the definition of the codes. Resetting the computer will not fix the car; unless the problems that triggered the light are fixed first. This should not be done until you find out what the codes are, and write the numbers down. After you find out what triggered the codes; the parts that are faulty will have to be replaced or serviced. And that is probably something that would need to be done by a shop that has experience, tools, and access to replacement parts.
If you cannot drive the car; you'll either have to have a mobile
mechanic come out to your home to repair the car; or have the car towed
to a shop.