If you don't know this simple information then take it into a mechanic. Don't risk breaking your system.

where is the spark plug in the toyota corolla 2003

I know this is an old question, but maybe someone else will need the info and end up here. When you pop the hood, you will see a black cover over the top of the engine. There are 2 acorn nuts toward the front and 2 plastic connectors toward the back. You need a 10mm socket to remove the acorn nuts and you can use pliers to remove the plastic connectors. Once the cover is removed, you will see 4 generally rectangular, pieces at the top of the engine. There is a four wire electrical connection to each one and they are attached with a bolt. Unplug the connector and loosen the nut with a 10mm socket. Pull up and twist on the electrical connection until it comes out. It's about 5-6 inches long and has the boot that fits over the spark plug. Once removed you can look in the opening and see the top of the plug. Using a long extension and a spark plug socket, remove the plugs. It really helps to have a good spark plug socket for this because the rubber fitting in the socket will hold the spark plug while you lift it out. When installing the new plugs, insert the new plug in the spark plug socket and lower it into the opening. I typically turn the socket counter-clockwise until I feel the lead thread and then clockwise to tighten. This is a simple DIY job. The original plugs on this car were made to go 100,000 miles, but if you are looking for this info now, chances are good that you have exceeded that mileage.