I hate to use a Wiki link, but they outline the process pretty well
http://en.wikipedia.org/wiki/United_...budget_process
United States Budget Process
The President, in accordance with to the Budget and Accounting Act of 1921, must submit a budget to Congress each year. In its current form, federal budget legislation law (31 U.S.C. 1105(a)) specifies that the President submit a budget between the first Monday in January and the first Monday in February. The President's budget contains detailed information on spending and revenue proposals, along with policy proposals and initiatives with significant budgetary implications.
The House and Senate Budget Committees begin consideration of the President's budget proposals in February and March. Other committees with budgetary responsibilities submit requests and estimates to the Budget committees during this time. The Budget committees each submit a budget resolution by April 1. The House and Senate each consider those budget resolutions and are expected to pass them, possibly with amendments, by April 15. Budget resolutions specify funding levels for appropriations committees and subcommittees.
Appropriations committees, starting with allocations in the budget resolution, put together appropriations bills, which may be considered in the House after May 15. Once appropriations committees pass their bills, they are considered by the House and Senate. A conference committee is typically required to resolve differences between House and Senate bills. Once a conference bill has passed both chambers of Congress, it is sent to the President, who may sign the bill or veto. If he signs, the bill becomes law. Otherwise, Congress must pass another bill to avoid a shutdown of at least part of the federal government.