President Trump has made a budget proposal — he is looking to increase military spending by 10% — which is a $54 billion increase — while offsetting the same amount of funds from non defense agencies. It seems like the Environmental Protection Agency (EPA) and foregin aid will have the most cuts. Most federal agencies will see an reduction in their budgets.
Social Security and Medicare budgets will be left untouched — Trump will not seek to increase nor reduce it.
The budget proposal “starts” the process to determine the budget for the next fiscal year, which starts on October 1. The proposal will be given to Congress, where they will start making a budget resolution and starts the process to set the federal budget for 2018, voting is expected sometime in the fall.
Clarification: The budget might not be signed by the President in time (by October 1). Congressional resolutions can be used to continue and fund agencies at their current levels.
Handy Guide to Federal Budget Process: http://www.washingtonpost.com/wp-srv/special/politics/federal-budget-process/budgetprocess.pdf