This is a hot topic for which you will find many answers; the best answer is firstly DO NOT handle wildlife injured or otherwise. If you happen upon an injured or lethargic wild animal the best thing you can do is keep distance and keep watch, contact a wildlife specialist as soon as possible. A wildlife specialist can evaluate the situation and explain to you if and how you should proceed with the animal.
Once you have contacted a specialist you may be asked to proceed in one of a few ways:
If the animal is small and determined to be non-dangerous you may be instructed to use a towel, blanket or gloves and gently move the animal into a safe, warm, dark contained location such as a box or carrier. Keep the animal in a warm dark location until a professional can retrieve it or until it can be taken to one. Never try to feed the animal as you can do more damage in many situations, you may be advised to provide a shallow water dish, generally something with water levels less than half the depth of the animals snout top to bottom is advised so they cannot drown themselves. Get the animal into wildlife care as soon as possible, the sooner they get care the higher the chance of successful recovery.
If the animal is determined to be dangerous you may be instructed to monitor the animal from a distance until a professional can be disbursed to your location.
If determined to be dangerous but safe enough they may ask you to contain the animal by placing something over top of it such as a box or empty garbage bin. In these last two cases you will likely be instructed not to attempt to give food or water as the specialists will be arriving to retrieve the animal sooner rather than later.
In all cases seeking the advice of a trained, licensed and well equipped specialist is advised. Wild animals are unpredictable especially when injured and scared. Do not risk injuries to yourself or the animal by taking matters into your own hands.