What should I do if I'm stung by a scorpion? Is the venom harmful to humans?
The answer is right there in your link. Some select tidbits to answer your question:
All known scorpion species possess venom
Of the 1000+ known species of scorpion, only 25 have venom that is dangerous to humans; most of those belong to the family Buthidae.
First aid for scorpion stings is generally symptomatic. It includes strong analgesia, either systemic (opiates or paracetamol) or locally applied (such as a cold compress). Hypertensive crises are treated with anxiolytics and vasodilators.
Unless you're an entomologist specializing in scorpions and you are damn sure the scorpion that stung you is not one of those dangerous to humans, you need to get medical attention ASAP. You probably won't have morphine or another opioid analgesic, but you might have a cold compress, so use it. As for vasodilators, you probably won't have any of them handy either, so, as I said, get to a hospital ASAP. And the advice in the snakebite question about lying down and trying to slow your pulse while someone (hopefully the third in your party, leaving one to look after you) runs to get help is a good idea.
Treat it like it is a snake bite. always have a snake bite kit with you when hiking and camping. As well as a basic first aid kit. Also have an Epi-pen handy.