Given that outlays are equal to taxes when the budget is balanced, what must the US government do to balance its budget?Economically speaking

You've just said what a government in general has to do to
balance a budge, so are you asking what specific steps the US government has to take in
order to make its budget balance?

If so, it clearly has to
A) increase revenues that it gets from taxes and/or B) decrease the amount of spending
that it does.  Practically speaking, the US government would have to do both in order to
balance its budget because the annual deficit is so
large.

As far as increasing tax revenues, most people would
say that the government needs to raise taxes.  However, there is also a school of
thought that says that lowering taxes will increase revenues as people work harder
because the lower taxes give them more incentive to earn.  (See the Laffer Curve link
for this theory.)

As far as lowering spending, entitlement
spending is clearly the major part of the budget and so any serious attempt at balancing
the budget would have to do something about entitlements.  (See the usconvervatives link
for a proposal.)