Assuming you're talking about a standard sized aerator, the process is actually quite simple and doesn't require any tools. All you need is a firm grip and a little elbow grease.
Start by unscrewing the aerator counterclockwise. You may need to use some force, but be careful not to strip the threads. Once it's loosened, remove it by hand and set it aside.
Underneath the aerator is a washer and rubber gasket. The washer is usually metal, and sometimes gets stuck in place. Use your fingers to remove it (again, being careful not to strip any threads) and set it aside with the aerator.
The last piece is the rubber gasket which sits between the faucet and the washer. This can usually just be pulled off by hand, but if it's stubborn you may need to use a flathead screwdriver or other tool to pry it loose.
And that's all there is to removing a faucet aerator!