Hi I am a student so you can't always believe everything I say but My understanding is in the simplest case possessive e.g. watashi no tomodachi which means my friend. Of course there are other uses of NO but why make it complicated at this stage. は which is pronounced wa is a topic marker e.g. ashita wa nichiyobi desu (as for tomorrow its Sunday) or Zo wa hana ga nagai desu (The elephants nose is long) As well as using this source why don't you use the app Memrise and use the minna no nihongo which is very useful I am sure there are more experienced people than me but hope that this straight forward but not complete explanation will be useful