All you gotta do to become a federal judge, from a district judge on up to the Chief Justice of the Supreme Court, is be nominated by the President and confirmed by the United States Senate. The Constitution sets forth no requirements for federal judges, not even minimum age, citizenship, or residency requirements. The Department of Justice, which reviews nominees' qualifications, has developed its own informal criteria. But the results of the Dept. of Justice review are just recommendations. The Senate, which has the ultimate decision, can ignore the recommendations. The Senators also have a set of informal requirements and more often that not their decision is based more on political considerations than on education or knowledge of the law or experience as a judge. Things are different at the State level, in Texas for example, Judges are elected by popular vote and the requirements to run for for the lowest level courts,District Courts, require US citizenship, 2 years residency in the state and district, minimum age of 25, and 4 years as practicing lawyer or judge. The requirements for Courts of Appeals and Supreme Court are similar with the minimum age raised to 35 and experience to 10 years. The Great State of Texas, in it's infinite wisdom, does have residency requirements for the higher courts, but forgot to set a minimum time, so if you get here 30 minutes before election filling time, you can run for Chief Justice. You don't even gotta be a registered voter to run for any state court. When you get down to county judges, Justices of the Peace, and such in Texas, there are no experience requirements at all, just a minimum 12 months in the state and 6 months in the district and to be 18 at time of taking office. I know of at least one Justice of the Peace who, when elected, was not old enough to take a legally binding oath of office, and a day laborer with an 8th grade education ran for JP an ran a close second to the winner. Various cities have various requirements but generally they are just the same requirements to vote in the city elections.

To add to notmrjohn's answer - Laws to become a circuit judge vary state to state. Several states require a judge to be an attorney that has been in practice for six (6) years before being able to run for judge on a ballot.