If you're an avid musician, whether professional or an amateur, having a MIDI keyboard is the way to go. With it you are able to create music that in the old days would require a good number of musicians, time and money in order to achieve the same results.
Nowadays most, if not all, digital or electronic keyboards are MIDI capable. You will know it because it should say so on their body and they should have the MIDI interface input and outputs in the back or side. That being the case you need to consider these before choosing your unit.
Budget: Before making any purchases ask yourself how much you're willing to spend. As with anything else the price will vary depending on the type, features and keys of the one you're planning to get. Obviously the more features and keys you get the higher the price.
Space: Another consideration before buying anything. If you're simply working from home then you may want to consider the size of MIDI keyboard you're getting - if you have a small space then smaller is better. Conversely, if you enough available space then get a bigger one of course dependent upon whether you can afford it or not.
Use: If you're a musician that's always on the road then perhaps lugging a 60-pound behemoth may not be the best thing for you. You may want to consider a smaller unit that's much more compact and portable - just what you need when you're running from one gig to another. If you're a stay at home kind of musician then bigger may be ok (if you have the budget and the space for it).
Keys: The number of keys determines the size of the MIDI keyboard. It comes in 88, 76, 61, 49, 37 and 25. You can even get them smaller than a 25. Piano enthusiast and hardcore composers would tell you that you should get the 88. While it may be a good idea the number of keys you get will depend on your budget, space and the intended use. As mentioned above you won't buy an 88 if you can't even fit it in your home studio. That will be like parking a tank in a space for a motorcycle - it ain't gonna happen.
Type: Do you want a controller type or a piano type. Controller types are typically smaller because they don't have speakers. That means in order to hear any sound you would need to have it attached to something else, like a computer with a Digital Audio Workstation (DAW) program. If you want to hear something as you play then get a piano type as it comes with internal sound system but this makes them bigger. (See budget, space and use constraints)
There are other things to consider before you get your MIDI keyboard - key feel, controllers, knobs, and brand - but these 5 are the basics before you purchase anything.
Check out useful tips in buying Midi Keyboards. Know what other users say in the Midi Keyboard Reviews section.
