Answer by taking a quote from the following article:

Evita is a musical with music by Andrew Lloyd Webber and lyrics by Tim Rice. It concentrates on the life of Argentine political leader Eva Peron, the second wife of Argentine president Juan Peron. The story follows Evita's early life, rise to power, charity work, and eventual death. The musical began as a rock opera concept album released in 1976.

In Buenos Aires on July 26, 1952, an audience is watching a film ("A Cinema in Buenos Aires, July 26, 1952"). The film is interrupted when news breaks of the death of Eva Peron, Argentina's First Lady, at the age of 33. The nation goes into public mourning as they sing "Requiem for Evita" (in Latin, which is modelled on a Catholic requiem). Che, a member of the public, marvels at the spectacle and promises to show how Eva did "nothing, for years" ("Oh What a Circus").  In 1934, 15-year-old Eva Duarte (later Eva Duarte de Peron) lives in the provincial town of Junin, and longs to seek a better life in Buenos Aires. Eva falls in love with a tango singer, Agustin Magaldi, after she meets him at one of his shows ("On This Night of a Thousand Stars"). Eva persuades Magaldi into taking her with him to Buenos Aires and though he is initially resistant, he eventually accepts ("Eva, Beware of the City"). Upon her arrival at the city, Eva sings about her hopes and ambitions of glory as an actress ("Buenos Aires"). After Magaldi leaves her, and Che relates the story of how Eva goes through several relationships with increasingly influential men, becoming a model, actress and radio personality ("Goodnight and Thank You"). He then tells of both a right-wing coup in 1943 and Eva's success, implying that Argentine politics and Eva's career may soon coincide. Che also makes a point to introduce the figure of Colonel Juan Domingo Peron, an ambitious military colonel who was making his way up the Argentine political ladder ("The Lady's Got Potential"). In a game of musical chairs that represents the rise of political figures, Peron and other military figures compete for power and exhibit their political strategy ("The Art of the Possible").  After the 1944 San Juan earthquake, Peron organises a charity concert at Luna Park to provide aid to the victims. Eva attends and briefly reunites with Magaldi, who spurns her for her past actions. Peron addresses the crowd with words of encouragement and leaps off the stage, meeting Eva as soon as he exits ("Charity Concert"). Eva and Peron share a secret rendezvous following the charity concert, where Eva hints that she could help Peron rise to power ("I'd Be Surprisingly Good For You"). Eva dismisses Peron's Mistress (the character is known only by that title), who ponders the rejection ("Another Suitcase in Another Hall").  After moving in with Peron, Eva is introduced to high society, but she is met with disdain from the upper classes and the Argentine Army ("Peron's Latest Flame"). In 1946, Peron launches his presidential bid after being promoted to general in the army, and while in bed with Eva, he discusses his chances of winning the election. Eva reassures him and soon they organise rallies where the people show their support and hope for a better future, while on the sidelines Peron and his allies plot to dispose of anyone who stands in their way ("A New Argentina").

What is act 1 about
In Buenos Aires on July 26, 1952, an audience is watching a film ("A Cinema in Buenos Aires, July 26, 1952").