As long as the bird is in no danger from cats or other predators, it is best to watch the bird from a distance to see if a parent returns after an hour or so before determining that the bird needs human help. Birds do not have a true sense of smell, so the myth that birds will reject a baby bird if touched by a human is not true. Second provide warmth to the baby bird. A hot water bottle or low heating pad or even hot water in a glass bottle will do. Taking care of injured birds and wildlife should be left to people who know how to do it best.
Bluebirds are typically insect and fruit eaters.
You can try feeding mealworms or Bluebird Choice to the bluebirds. Place a small amount of mealworms on the ledge of the bluebird feeder until the birds get used to finding the mealworms. The bluebirds will learn to go into the feeder for mealworms. One of the most effective ways to keep squirrels from feeders is to place a baffle on the pole or above the feeder and situate the feeder so the squirrel cannot reach it. A baffle will make it harder for the squirrels to reach the seed.