While there may be no specific standards that a judge must meet in order to be appointed, the Presidents in past years have typically gone with federal judges and former lawyers. The higher the qualifications and the better the resume the better the chance that the Senate allows the appointment.
What the judge believes in is especially important when the President considers who to pick. Naturally the President is going to want to have as many people who are in parallel to his beliefs around him as possible so to be able to have judges, who ultimately decide the law of the land, by his side and having the same agenda, would be extremely helpful in running the country.
If the President has low popularity and tries to push a judge in accordance to his ideals, it will end up hurting his popularity even more, destroying his and his party's chances for re-election. If popularity is low, then the President may choose to appoint a more moderate judge.