Firstly you must have the following:
Cauldron (Crafted with 7 Iron Ingots)
Brewing Stand (Crafted with 3 cobblestone and 1 blaze rod)
Glass bottles (Crafted with 3 glass)
Nether Wart (Found in the Nether)
Magma Cream (Dropped by Magma cubes)
With the Cauldron full of water, fill the glass bottles to turn them into water bottles (3). Now open the brewing stand and place your 3 water bottles into the stand and place Nether Wart at the top and let it brew.
You should now have 3 Awkward potions, now place Magma Cream at the top and let it brew again which will give you your Fire Resistance potions.
Now simply drink one of the potions, find some lava and swim!