Answer by taking a quote from the following article:

Cher (; born Cherilyn Sarkisian (Armenian: Sherilin Sargsyan); May 20, 1946) is an American singer and actress. Sometimes called the Goddess of Pop, she has been described as embodying female autonomy in a male-dominated industry. She is known for her distinctive contralto singing voice and for having worked in numerous areas of entertainment, as well as adopting a variety of styles and appearances during her six-decade-long career.

In early 1965, Caesar and Cleo began calling themselves Sonny & Cher. Following the recording of "I Got You Babe", they traveled to England in July 1965 at the Rolling Stones' advice; Cher recalled, "[they] had told us ... that Americans just didn't get us and that if we were going to make it big, we were going to have to go to England." According to writer Cintra Wilson, "English newspaper photographers showed up when S&C were thrown out of the London Hilton [because of their outfits] the night they arrived--literally overnight, they were stars. London went gaga for the heretofore-unseen S&C look, which was neither mod nor rocker."  "I Got You Babe" reached number one on the Billboard Hot 100 chart and became, according to AllMusic's Bruce Eder, "one of the biggest-selling and most beloved pop/rock hits of the mid-'60s"; Rolling Stone listed it among "The 500 Greatest Songs of All Time" in 2003. As the song knocked the Beatles off the top of the British charts, English teenagers began to emulate Sonny and Cher's fashion style, such as bell-bottoms, striped pants, ruffled shirts, industrial zippers and fur vests. Upon their return to the U.S., the duo made several appearances on the teen-pop showcases Hullabaloo and Shindig! and completed a tour of some of the largest arenas in the U.S. Their shows attracted Cher look-alikes--"girls who were ironing their hair straight and dyeing it black, to go with their vests and bell-bottoms". Cher expanded her creative range by designing a clothing line.  Sonny and Cher's first album, Look at Us (1965), released for the Atco Records division of Atlantic Records, spent eight weeks at number two on the Billboard 200, behind the Beatles' Help!. Their material became popular, and the duo successfully competed with the dominant British Invasion and Motown sounds of the era. Sonny and Cher charted ten Billboard top 40 singles between 1965 and 1972, including five top-ten singles: "I Got You Babe", "Baby Don't Go", "The Beat Goes On", "All I Ever Need Is You", and "A Cowboy's Work Is Never Done". At one point, they had five songs in the top 50 at the same time, a feat equaled only by the Beatles and Elvis Presley. By the end of 1967, they had sold 40 million records worldwide and had become, according to Time magazine's Ginia Bellafante, rock's "it" couple.  Cher's following releases kept her solo career fully competitive with her work with Sonny. The Sonny Side of Cher (1966) features "Bang Bang (My Baby Shot Me Down)", which reached number two in America and became her first million-seller solo single. Cher, also released in 1966, contains the Burt Bacharach and Hal David composition "Alfie", which was added to the credits of the American version of the 1966 film of the same name and became the first stateside version of the popular song. With Love, Cher (1967) includes songs described by biographer Mark Bego as "little soap-opera stories set to rock music" such as the U.S. top-ten single "You Better Sit Down Kids".

What was their first hit?
I Got You Babe