Chances are the Mom is near and possibly out hunting or looking for food or water. Keep an eye on him or her but leave them where you find them. Kittens have a lower survival rate if they do not have a Queen (Mama cat) to take care of them. Queens provide all the nutrients the kitten will need to survive with her milk. However, if it is a newborn and it has been 3 hours or more, he/she may need medical attention. Best bet is to take them to a vet. However, if it is after hours or there is not a vet nearby, try to keep the kitten warm. It is very easy for a kitten to not make it if he/she is out in the elements and not kept warm. 

At this point they will require feedings every 1 to 2 hours and be stimulated to go potty. This is a big undertaking and preferably a trained bottle feeder will be needed. If you think you are up to the task, you can buy formula (from just about anywhere). Make sure its kitten formula NOT  cows milk. Again, its not the queens milk but its the best replica. You will need bottles and either tissue or cotton balls to stimulate before and after meals.  For even more info and help and support please go to https://www.facebook.com/groups/kittenfostersupportforum/  This is a Facebook group run and managed by SD Kitten Rescue. There are thousands of experienced bottle feeders that can offer helpful advice as well as legitimate rescues and shelters that may be able to direct you to a local facility. We feel that knowledge is power and we are stronger together. Again, best bet is to take them to a vet, but there are times when its necessary for human intervention.