Bike maintenance questions.....

I've never maintained my own bike and I'm worried i'm wearing something out. From what I understand, this is great for the chain, this is great for putting in between the cracks of the derailleurs and mechanics...

Do i need to put something on the chainrings or cassette? or does the chain oil suffice? IF so, which should I put on the chain ring?

Also, is it necessary I use a degreaser to get rid of the current grim? Can't I just rub off the grim? Can I use WD-40 as a degreaser?