That depends on what type of connectors your MP3 player and stero have. If your MP3 player uses a standard 3.5mm headphone jack and your stereo uses RCA connectors, you should use a 3.5mm to RCA cable. These can be picked up at almost any dollar store or at Radio Shack. If your stereo only has a 3.5mm microphone jack, you'll need a 3.5mm to 3.5mm cable. These are slightly less common but should still be available at many electronics stores.