As we have seen recently US presidents are able to issue executive orders which have the force of law however they are subject to judicial review and may be struck down if deemed by the courts to be unsupported by statute or the Constitution.
To learn more about executive Orders see this explainer from the Conversation
For fuller information the Legal Institute at Cornell University has a good dictionary entry
They are printed daily in the Federal Register. The National Archives site explains the process and has tracing guides.
CNN is updating an easy understandable list of President Trump's executive orders on its politics webpages.
The Great American Presidency project also has the text of all orders from 1826 onwards
See this order on the funeral of Adams from 1826.
The site also has a wealth of other information on historic and current presidents including texts of proclamations
finally for further information
The Law Librarians Society of Washingon DC has a set of links to all kinds of explainers on the process