You wouldn't by any change be taking on developing apps for iOS -- the iPhone, iPad? I'm asking because I've personally only heard of Objective C being used for iOS apps. This is important, because if are then you must have a Macintosh.
If I'm wrong, ignore my answer and carry on.
Look, my original answer was deleted because it wasn't recommending a specific product. I understand those are the rules of this forum, but if my above assumptions are correct, them the information provided in my answer is vital, since buying anything else than a Macintosh (like the accepted answer recommends) is misleading and will only result in you wasting your money! I'm not a Mac person, but here, because rules, is a randomly recommended specific product that'll actually let you compile and run Objective C as you required. You want at least 8GB of RAM.
On a side note: You may be interested in knowing that all iOS developers (that I know of!) are moving away from Objective C and switching to the Swift programming language.