You need an AV receiver. This will receive digital audio connections (like fiber optic or digital coax) as well as your composite audio connection (white/red RCA). The AV receiver will also power your speakers.
Look on the back of the speakers and check what their impedance is. Most speakers are 8 Ohms, some are 6 Ohms. Most AV receivers will be set to 8 Ohms by default, but can be adjusted to 6 Ohms. If your speakers are a different impedance, then it is likely they are part of a "in-the-box" system. This would mean that they are part of a proprietary system and were probably powered by an AV Receiver/DVD combo of some kind.
Without any more information, this is my best guess for you. If this doesn't sound right or doesn't help, please post the make and model of your speakers and possibly some photos of the equipment. The more information you provide, the better we may assist you.