What is the best thing to do for an infection undet the skin by a fingernsil?

Hot soaks: You can try to treat it by putting the finger in hot water at the limit that you can stand for a few moments, dipping it in and out until the water cools. Do this 4-5 times a day and most of these will clear. There is an infection next to the nail termed a "felon" which might require a hand surgeon to open and drain. This would be the next stop if the heat does not work.