What Do I Do If I Am Bitten By A Rattlesnake?

First, the severity of symptoms from a rattler bite depends on how much venom is injected. Adult, experienced-hunter rattlers do not inject all of their venom, unlike juvenile snakes.
First, do not panic and move around quickly as this will increase the flow of the neurotoxins in the venom. Lie down and keep the affected area above the heart, if possible. I once heard a park ranger say the most important factor in treating snakebite is your car keys: send someone for help or drive yourself to a medical facility immediately.
Healthy adults will not die of a rattler bite, but you will experience a lot of pain. The bite area will become painful, discoloured, and swollen. As the poison sets in, you will have trouble breathing and become nauseated. The flesh around the bite area may become necrotic and black.
The only effective treatment for rattler bites is (very expensive) antivenin, but it can cause severe side effects. But be forewarned that once administered, it will never again be effective. Traditional bite treatments like tourniquets, cutting the bite area then sucking out the poison, and electroshocking it have all been proven ineffective, and may cause more harm than good.
I met a guy in Arizona who was a self-taught expert on a particular species of rattler. He had been bitten on his hands seven times, but only sought medical attention once  -- after his roommates freaked out. He would take aspirin to thin his blood then lie down and wait out the extremely intense pain, which lasted 24 hours. I examined his scar tissue and it was normal, not black.