As you exit the Temple of Time, you find that Hyrule Castle Town is in shambles and there are enemies all throughout. These are ReDeads, who are like zombies. If you get close enough and they see you, they can freeze you in your place and try to grab onto your back, which will start depleting your life. Hyrule Castle has been replaced with Ganon's Castle, but there's nothing you can do here for now, so exit to Hyrule Field.
Your goal for now is to make it to the Forest Temple and complete it to find the Sage there. However, there are a few stops that are needed on the way, as you won't be able to get into the Forest Temple right away. As Sheik mentioned, you should visit Kakariko Village first, but there's one optional spot that should be visited before that.