Yes, you will need MIDI if you want to use the keyboard as a controller for GarageBand. Select one which uses a USB cable for MIDI rather than the old 5-pin DIN connectors, then you won't need an adaptor. I think most modern MIDI keyboards have USB these days, but make sure. Other things to consider:
- Do you need full size keys, and how many octaves do you want? i.e. are you already, or intend to become, a very good keyboard player, or do you just have basic skills. If you're very good, or intend to develop your keyboard playing skills, then you probably want full size keys and at least four octaves. If you're not a very good player but just want to play melodies and basslines separately into GarageBand then a smaller keyboard with mini-keys may well suffice.
- Do you need the keyboard to produce sound independently? i.e. do you want to use the keyboard separately like a piano/organ or do just need it to control GarageBand or other external MIDI devices. If the former then look for internal sound generation and built in speakers.
A mini-keyboard that's just a controller will obviously be much cheaper and can be had for £60 or £70 (here in UK). A full-size keyboard with on-board sound production and built-in amplification and speakers will obviously be more expensive.
If you just want a controller keyboard then this is one that I can recommend:
http://www.amazon.co.uk/Akai-MPK-mini-MPKmini-controller/dp/B00466HM28/ref=wl_it _dp_o_pC_nS_nC?ie=UTF8&colid=32QUXGNOH1T7Z&coliid=I2E8XQG8A01YOQ
It's modestly priced and comes with the added benefit of tap-pads that you can use for entering drum/percussion parts into GarageBand.
