Rule out injuries ... then find mama.
If the baby squirrel doesn't have visible wounds and seems warm, the main goal is to get the baby squirrel reunited with his mom if at all possible. Usually, the baby has just fallen from the nest.
"We ask the people to make a little nest box, to put them where the mother can see them, and take them up to the nest again." A nest box is a container that can be attached to a tree where the mom can find the baby and bring him back to the nest. (You can make or buy one of these - or even use a makeshift cardboard box.) If you don't know which tree the baby fell from, just choose the tree closest to where you found the baby.