What does Sam realize he needs to survive winter in My Side of the Mountain?

Sam realizes he needs plenty of wood as well as food before winter comes.

Since Sam has decided to live alone in the wilderness, he needs to ready his home for the winter.  He hunts many different kinds of meat, and finds a dead deer that a poacher killed to make warm clothing and a door for his tree.

During the summer and fall I had thought about the coming of winter.  However, on the third day of December when the sky blackened, the temperature dropped, and the first flakes swirled around me I must admit that I wanted to run back to New York. (ch 1, p. 7)

Sam needs a variety of things to survive winter.  He needs a good house that keeps the snow out.  He realizes he needs dry wood to build fires.  He needs smoked fish and dried meat that he can eat when there is no way to hunt food.  Somehow, Sam is able to do all right.

The wood is important because in the winter it is hard to get dry wood, which is needed to start a fire.  The fire not only keeps Sam warm, but cooks his food.

What crucial thing did Sam forget in his winter preparations in My Side of the Mountain?

Sam's taken a lot of time and trouble to prepare for life in the wilderness during winter. It can get pretty cold in the Catskills at this time of year, so Sam knows this will be a real challenge, the ultimate test of his survival skills. Yet because there are so many things he needs to remember as part of his preparation for winter, it's inevitable that Sam would forget something and that that something would be very important indeed.

It's only when Sam returns to his tree after paying a visit to the town of Delhi that he realizes what he's forgotten to do: stack up a big pile of firewood. It's amazing that Sam should've overlooked something so basic; he'll need wood—and plenty of it—for keeping warm and cooking, right throughout the harsh winter months.