Have you gotten to Spire yet? If not, do the following:
1) Take the top exit out of Khur and go through the Magma Tunnel, then go all the way to the bridge to the volcano at the top of the world map.
2) Once you're there (The map should change to Lord's Bridge), progress through a few cutscenes and go through Spire until you reach the Obsidian Lords' throne near the top. 
3) After talking with the Obsidian Lords near the top of Spire, you should be told to go to the neighbouring beach town of Old Nyaren and talk to someone there named Riley about getting a boat. You can find her in the Alchemist's Lair opposite the water.
4) Once Riley's agreed to let you use her boat, you'll have to take the boat to Blightwood, where you can ally with either the angels or Fire Elementals in order to gain the trust of someone who will eventually get you into the Fire Temple.
Getting to the Fire Temple itself will take a while, but start with the steps listed above and you should have no problem progressing to that point!