To craft a fire resistance potion, you need to go to the nether and obtain magma cream by killing a magma cube and a nether wart. Then add the ingredients at a brewing stand in this order:
Water bottle
Nether wart
Magma cream
Don't forget you need blaze powder to fuel the brewing stand. Drink the potion, then jump in lava. This is easiest to do in the nether since lava is everywhere.