If you've programmed in C you can learn Objective C very easily. I learned to program the Mac with Objective C and Cocoa by reading "Cocoa Programming for Mac OS X" by Aaron Hillegas and Adam Preble. If you read this and do the examples you will be able to write a full-featured Mac application. This is out of print but you can probably find a copy on Amazon or Ebay. I have the fourth edition. They stopped updating it when Apple came out with Swift, probably because of the difficulty of maintaining it for two languages. I haven't looked at their site, Big Nerd Ranch, in a while. It's likely that they have other useful books for you.