There are formal and informal qualifications to run for U.S. Congress.
The formal ones do not change, they are age ( 30 years old for Senate ), American citizenship and a residence in the state in which the candidate is running.
As for informal qualifications, they change according to new political emergencies in the political system and the overall political and economic situation in the country and abroad.
Some of the most sought of informal qualifications are: education and knowledge about national and international events and policies, an interest in history and social studies, an emphatic drive to help people and serve the country. Good communications skills are always an asset as well as the ability to debate well. Certain standards of ethics and morality should always be maintained by all the candidates, mainly honesty and integrity. The candidate should also have the ability to raise money to fund his run for office.