Besides microphones, good monitor speakers are the most important tools in your studio. Over my 25 years selling and using monitors, I've learned some basic truths that will help you get the most out of your speakers and choose a pair that will enhance your mixes, and bring joy to your ears.
1) How to buy. Try before you buy! If at all possible you need to hear them first. Establish a price range you can afford and listen to as many as you can. One pair is going to stand out and make you go "ooh ah." You really should love listening to them. THEN you need to learn how your speakers translate to the rest of the world. How much bass do you put in your mix? You'll mix the opposite of what you hear i.e. if you don't hear much bass from your speakers you're likely to add more bass to your mix to compensate and vice versa. The only way to deal with this is to compare your mixes on as many other speakers as you can. Over time you can learn how your mixes translate so your results will be consistent and pleasing. Try to avoid using mp3's to audition speakers. They will never give you the detail and clarity you get from a CD. It's like looking at the world through a screen door. 2) Positioning. Unless you do only mono mixes---hey the Beatles did for years---you'll be able to hear the panning and depth of your music best if your monitors and your head create an equilateral triangle. Get the tweeters head high. In the confines of most small home studios it can be hard to get your speakers away from the wall, but if you can keep them at least 18 inches away from the wall behind them, you'll hear better imaging. Imaging means each element of your mix sits in the space and you get a sense of where the instrument sound is coming from, front to back as well as side to side. 3) Speaker stands. Another thing that can be effective in improving the quality of the imaging is to de-couple your speakers from their stands. Mouse pads, Auralex Mo-Pads or even the way more expensive products from Primacoustic and others will isolate your speakers from whatever they are sitting on. De-coupling matters because if the speaker is transferring it's energy to the stand it's doing a less efficient job of moving air and creating sound. Sand filled stands are great. Stands should be dense and not contribute wayward vibrations to the sound. I've seen people use concrete culvert pipe! Overkill? Maybe but boy did they sound good.
4) Sub-woofers Subs can supply the oomph in a kick drum or the giant energy of a bass synth. Drama! If you do hip-hop or dance music you will need a sub. Cheap subs can sound muffled, frumpy, or poorly defined. Save up for a good one and you'll be better off. Bass sounds best when it's tight, clean, and LOW! If you don't need tons of bass, and you have a monitor with an eight inch woofer (or bigger) you can get away without one. Five inch woofers and smaller rarely create the bottom octave very well, but often budget or space restraints will not allow the use of a larger speaker, so if that's the case, just learn to translate as stated above. You can! I'm only going to recommend two monitors here---and this is only my opinion of course---BUT Presonus has been quietly making the Eris 5 and Eris 8 ($300 and $500 the pair) for a long time now and it's hard to find something better for the money.