Projecting the National Debt
Understanding what makes up the present national debt is discussed in "How the U.S. Works". Because future level of national debt is affected by future events, you need to make assumption about what future events will happen. Once you have your set of predictions, you calculate the future debt like you calculate present national debt. The accurateness of your assumptions will determine the accuracy of your prediction.
Some of the events you would need to project are things like The 9/11 event, war, new legislation, changes in taxation, War on Terror, interest rates, international and domestic spending, etc.
Some groups that do make predictions are The Congressional Budget Office, The Center on Budget and Policy Priorities, The Committee for Economic Development