Objective C has a way of setting up getters and setters for you so that you don't have to worry about writing them yourself. It does this through properties. To set up a property, in your header file write
@property (strong, nonatomic) NSString *songID;
Typically, you will use the strong and nonatomic modifiers unless you have a reason not to. You can research other parameters if you're interested. After writing that line, the getters and setters are generated for you.
To use this property within the class, you simply use dot notation and the self object. For example, to set the value, you write
self.songID = @"66777888";
You access the value of the property in the same way. For example, to assign another string equal to the value, you write
NSString *other = self.songID;
If you want to access these values outside of the subclass, you still use dot notation, except now using the instance of the class. For example, if you want to set the songID from another class, you can write
Song *song = [[Song alloc] init];
song.songID = @"790";