Amps and receivers are normally rated based on the number of watts (power) PER CHANNEL. In addition, you need to consider the impedance of the speakers you will need. To be able to select speakers for your amp, you need to find out 3 things about your amp:
- How many channels does it have?
- How many watts per channel?
- What impedance does the amp support?
Stereo amps have only 2 channels and you cannot drive more than 2 speakers. Technically, you can connect more than 2 speakers to a stereo amp but this is not what you are looking for. Home theater amps are typically 6 channel or 8 channel. A 6 channel is also known as 5.1 (2 front main speakers, 2 rear surround speakers, 1 center channel speaker and a subwoofer). An 8 channel simply adds 2 more surround speakers to the 6 channel setup.
I'm not aware of any consumer-level home theater amps with 250 watts per channel, so my guess is that you have a stereo 2 channel amp with 125 watts per channel. You cannot use this for a home theater setup with more than 2 speakers if that is the case. If you don't know the specs of your amp simply search for the model number in Google... usually you can find the specs that way, or alternatively you may be able to find the specs or manual on the manufacturer website.
Most multi-channel home theater amps are rated around 100 watts per channel, so you are looking for speakers with a rating close to that. Speakers also have an impedance rating in ohms you need to consider. For home speakers this is usually 8 ohms, but some are rated 4 or 16 ohms. The rule for impedance is never use speakers with a lower impedance than your amp supports. In other words, if your amp supports 8 ohms, never connect speakers to it that are rated for 4 ohms or you may damage the amp. It is safe to connect speakers that are rated HIGHER, but they will not be as loud. In general, if your amp is rated for 8 ohms then you need to get speakers rated for 8 ohms.
Don't worry too much about speaker wattage ratings. Try to get speakers that are rated close to the output power of the amp but it is not that critical. Obviously you wouldn't want to drive 5 watt speakers with a 150 watt amplifier at high volume, but don't worry too much if the speakers are rated slightly below your amp. You will not damage the speakers, in fact it is more common to damage speakers by trying to drive them with too little power than too much. Trying to run speakers with too little power at high output levels causes distortion (called clipping) of the signal going to the speakers. Clipping is the leading killer of speakers. You may or may not hear clipping, depending on your hearing and sensitivity to it, but if the sound becomes distorted at higher volume then it is most likely the amp running out of power and causing clipping.