Learn C language first. Because most of the low level computer programming stuff might be covered by that and you may get to learn how to program with much primary capabilities provided within the language. Because this is mostly Procedure Oriented Programming (POP).
Then move on to learn Objective-C which is an Object Oriented Programming (OOP) Language. Here you can use the skills you used to program using that primary language and you might feel that you are provided much power within the Objective-C and also be able to describe and make use of world in more Object-Oriented manner!
And must remember, as you are going to implement iOS Applications, you better have a good knowledge about Objective-C before moving to learn Apples new language for its products like Mac OS X and iOS, which is Swift! Because Objective-C has been used by them for a long period of time, and still it's been used by them.
Here read the following question and it's answers if you want to clarify more:
Learn C first before learning Objective-C
Hint: Seems like your Question also might be closed sooner enough saying it's been duplicated or answered before! :-)