Through the annual federal budget process, U.S. Congress passes laws making money -- lots of it -- available to the various government agencies for doing major projects designed to assist some sector of the public. The projects may be suggested by the agencies, Members of Congress, the President, states, cities, or members of the public. But, in the end, Congress decides which programs get how much money.
Once the federal budget is approved, funds for the grant projects start to become available and are "announced" in the Federal Register throughout the year. Grant projects that have been announced will appear in the Catalog of Federal Domestic Assistance (CFDA). The CFDA is a listing of all grant and assistance programs (currently over 1,420) administered by 57 federal agencies.
Newly available grants programs are also announced in the Weekly Federal Funding Report, published by the House of Representatives and in the Federal Register as a Notice of Funding Availability (NOFA).