What was the first action taken by the congress?

The first action taken by Congress was that of passing an act to regulate the time and manner of administering certain oaths.

Explanation:

New York City was chosen as the temporary capital of the United States and the first Congress met here in March 1789. The issues on the unwritten agenda were that of creating a treasury, a war machinery, a foreign affairs department, a sound tax system, a strong judiciary, etc. All of these issues were considered by the Congress one by one and the offices for the same were established by passing necessary laws through the next few meetings of the Congress.