You're going to need to learn Objective-C in order to write Cocoa apps. Some people say you need to learn C first, then Objective-C, while others say you can jump straight into the Objective-C (which is kinda how I did it). For learning Objective-C, I would recommend Kochan's "Programming in Objective-C 2.0" which is very well written and assumes no prior programming knowledge.
Teaching yourself Cocoa from scratch is a very challenging process, but is doable if you stick with it and work hard. Cocoadevcentral (linked above) is a great resource for someone just starting out. You might want to go through a few of the tutorials just to get a sense of whether this is something you're going to enjoy learning/doing because it is a pretty big commitment.