First up you need to get yourself familiar with algorithms and data structures - for that it doesn't really matter what language you use - C or Java would be suitable.
You might think using Java to start with is nonsense as you want to continue to Objective-C, however, it is simple to use and has great object-oriented capabilities.
Of course it's your call, you can use C to start with - as long as you get familiar with data structures, input/output, coditionals, loops and what I like to call "bugging the user"
- use EXAMPLES!
When you get familiar with C-like syntax you can move onto more complex structures, objects, composites, frameworks -- you are bound to come across advanced topics everywhere
Personally I started with Java, then went to C where some people have a hard time with pointers (!! very important for C itself, very powerful), then some Bash scripts and never really got to working with Objective-C ... bummer
Most important - use examples and write your own code, don't just read code, write it!
And have fun