In light of the April 15 protests, I think this chart by the Center on Budget and Policy Priorities which describes the federal budget is appropriate:

You’ll notice that the largest component of the budget is defense spending (it narrowly edges out Social Security), and unlike Social Security, there is no designated revenue stream (which incidentally has a surplus). Any discussion of budget balancing that does not consider defense spending is simply not serious. The other thing to note is that this does not include tax expenditures, the largest being the employer healthcare tax deduction of $134 billion.