To find the correct replacement cartridge, you need the faucet brand and model number. If that information is unavailable, remove the old cartridge and take it to the store. Use the store's reference guide to find the kit that contains the matching replacement cartridge and replacement rubber O-ring seals.
If the store displays repair kits on a wall you can use your faucet make and model number or the old cartridge to help find the replacement. It may help to take a photo of the faucet to the store as well so that a store associate can help find the exact cartridge.