How to effectively navigate API reference for IOS 9

Hi ,

I see that there are inbuilt variables for IOS
Ex : NSIndexPath

When one programs how do you navigate the API reference ? Secondly which API reference do you use to discover these interface variables ?

I’m discovering some variables through tutorials but when i practically try to implement a program i may need more complex variables.Unsure what would be the effective method of discovery.

I’m new to programming and programming with IOS/swift, apology in case i’m not using the right language to express my concern.

Appreciate your response.
Thanks.