Now you can head to the Temple of Time, which is located in Hyrule Castle Town. Once you enter it, head to the altar and read the inscription, which will say to play the Song of Time. Do just that and the three Spiritual Stones will appear and place themselves in the altar, with the Door of Time opening as well. Go inside and Navi will point out that the sword is the Master Sword!
Go up and grab it, which will trigger a rather lengthy cutscene that starts out with Ganondorf following you into the Sacred Realm. When you wake up, you'll find that seven years have passed and you're standing before the Sage of Light, Rauru. He will tell you that Ganondorf has obtained the Triforce and spread his evil across the land. It is your duty to find the other Sages and put an end to the rule of Ganondorf.
As you're transported back to the Temple of Time, Navi will mention that because of your new size, you won't be able to use some of the weapons from when you were a kid. When you try to exit, you're stopped by Shiek, one of the Sheikahs, who explains that your first goal is to awaken the Sage in the Forest Temple, but you should head to Kakariko Village first.