4 cups white sugar
juice of one lemon
Add Sugar!! You also need to add the lemon juice at this time, this keeps the fruit from turning brown and adds a nice tartness.
(Some people will add pectin during this stage but fruit makes it's own pectin when cooked with sugar, so I don't think you need it. It is a binder and causes the jam to gel faster. If you have a bit of time and patience you will get jam soon enough without pectin.)
Let the peaches sit for an hour which will cause the juices to flow.
Bring to a boil on medium heat then turn to a simmer on low for the next two hours. Stir occasionally.
As the peaches cook a few of the smaller pieces will disintegrate and the syrup will thicken. The longer you cook it, the more it will reduce and the thicker the jam will be.
National Center for Home Food Preservation. They are a wonderful resource on canning and food preservation.
Refrigerate after opening.