Fire Damage Bar

I am working on a new type of system using a fire damage bar. The idea is when you take damage, for every point of damage you take whether from fire or lava, you gain a level of fire damage up to 20 meaning that the bar's initial amount is 0. I've looked at some videos for mana and thirst bars and tried to go from there, but I have been so far unsuccessful. If anyone can help with this, I would appreciate it.