The most common piece of advice concerning speakers is "buy the best speakers that you can afford". But what exactly makes the best speakers for your own audio or home theater system?
A good place to start trimming down the field of offerings is by choosing the speakers that will fit the size of your amplifier or receiver. A receiver is nothing more than an amplifier with a radio tuner attached to it, so I'll continue to use amplifier to refer to them both. An amplifier always has rated power/channel, and this is the technical specification that you want to write down and memorize before you go out shopping for new speakers. This spec should be located on the back of the amplifier unit near the speaker connections. For example, my Yamaha 2 channel amplifier is rated at 50W per channel at 8 ohms. That's how much electrical power on average will be flowing into my speakers at that particular resistance level. So I want to look for 8 ohm speakers that have a rated power handling of at least 50W, and cut out the rest of the speakers as being unqualified. If I try to use less resistant 6 ohm or 4 ohm speakers, I risk blowing my amplifier if I turn the volume up high. And if I try to use speakers that have a smaller power handling of less than 50W, I risk blowing my speakers if I turn the volume up high.
Once the selection has been trimmed down due to technical specifications, you'll need to decide if you want one-way, two-way or three-way speakers. One way speakers have just the midrange woofer, no tweeter or subwoofer. Two-way speakers have a tweeter and a midrange woofer, but no subwoofer. And three-way speakers have a tweeter, a midrange woofer and a subwoofer. Your budget and your personal listening taste will help you to determine which type and brand of speaker is best for your own sound system from there.