You can create a property list file (there is a template for that in XCode, just to to File -> New file and choose there), so you will have something like “settings.plist”, or anything like that. You can think of a plist as being a key => value config file, with the difference that you can also hold arrays and dictionaries, besides plain text as value.
Use NSBundle to load the file in your app, like
NSString *path = [[NSBundle mainBundle] pathForResource:@"settings" ofType:@"plist"];
NSDictionary *settings = [[NSDictionary alloc] initWithContentsOfFile:path];
You can then access your keys as a regular dictionary. Just don’t forget to [release] it after ;).