How do i change my own oil in my 2008 grand prix

Raise the car.
Put a pan the holds at least 2 gallons under the oil plug
Loosen and remove the bolt in the very bottom of the oil pan and remove; the oil will flow out and into the pan.
Clean and replace the oil plug, tightening it snug. the plug has no load on it, you don't have to tighten it really tight. Tight enough to stay in and seal against the oil is all that's required. Of course, you also want to enure it doesn't fall out...
Unscrew the oil filter. (Use the appearance of the new one you bought as a guide for what you are looking for.) The filter is often difficult to remove. There are special tool for removing them — you should probably get one. DO NOT USE AN OIL FILTER WRENCH TO INSTALL THE NEW FILTER.
Dump the oil ou of the old filter sufficiently to handle it without sloshing.
Put the mounting surface of the new filter and the old filter together to ensure that the rubber rings are the same. Better to find out you have the wrong filter now that after you screw it on, and have your new oil leak out.
Pour some of the new oil into the new filter. Starting the engine will fill it up, but the longer it takes to fill, the longer the engine runs without a good supply of oil. The pre-filling you do shortens that period.
screw the new filter into place. Carefully find the point where the rubber sealing ring makes contact, and tighten the filter 1/4 turn farther. Do not over-tighten the filter.
Remove the oil filler cap on the top of the engine, and pour in 5 quarts of oil.
Look to ensure no oil is leaking. I once forgot to replace the oil plug and poured 5 Qts trough the engine onto the ground. That is bad enough. But if I had not checked before I started the engine I might have destroyed the engine as well..
Check the oil dipstick to ensure you have enough oil in the engine.
If no leaks, and enough oil on the stick, start the engine without using the gas pedal — just let it rise to an idle on its own.
Look under the car for any leaks. If the oil filter is the wrong one, or is improperly installed it will leak right away.
If no leaks after 1 or two minutes, lower the car and shut the hood.

Please dispose of the used oil responsibly.