Most aerators (located under the faucet) can be taken out by hand by just unscrewing it from the faucet. If it's in there tight, wrap some masking tape around it and grip with pliers unscrewing it clockwise.
Disassemble the aerator components, If the components are stuck, soak the aerator in white vinegar or lime-dissolver solution (one part vinegar to one part water) for 30 minutes. Take an old toothbrush and scrub it clean and then pop it all back on. You can screw it back in by hand or user your piers to tighten it.
Doesn't have to be too tight!