How do you change the oil on a 2005 Pontiac Sunfire?

This is a pretty basic job, but it's really a lot easier to find a shop that's offering a sale on oil changes.
But, if you're determined to do it your self, you might want to pick up a shop manual for your car, just to look at the photos to get an idea what you're dealing with and to read their procedures. The oil change entails removing the oil pan drain plug and letting all of the old oil drain out into a proper catch pan. (pick one up at the parts store. You can use it to take the old oil back for recycling)
You also need to replace the oil filter. You'll probably want to pick up an oil filter wrench at the parts store too. The filter unscrews from the mount. The new one screws back on (hand tighten only. Don't use the filter wrench!) after you put a light coating of fresh oil on the rubber gasket on the end of the filter. Filter manufacturers suggest not over-tightening. Read their directions!
Once all the oil has drained, replace the drain plug (make sure it's free of dirt or other crud) and snug down with a wrench. Don't go crazy here! Fill the engine with oil (4 quarts in a 2.2 and 2.4 OHV engine, 5 quarts in the 2.2 OHC engine)
When you have the drain plug in place, the oil added and the oil filter changed, start the car. Check for any leaks at the drain plug and at the oil filter. If you see no leaks, you're done! If you DO see leaks, tighten the plug or the filter slightly until it stops
Last step, drink a cold beer and know that your car is really going to like you for changing her oil ;)