Your questions has two distinct parts. First, you ask about the reasons for American entry into World War I (1914–1918). Second, you inquire about changes in America as a result of the war.
America did manage to stay out of the war up until its final year. This reluctance to enter a European war had been a long-adhered to practice dating back to the nation's first president, George Washington. The US did finally enter in 1917, though. Why was it unable to remain neutral? There were two primary reasons for America's eventual participation. First, America sought to uphold the principle of freedom of the seas.
While both sides interfered with freedom of the seas, the US was really only concerned with German interference using deadly submarines. Germany sank the Lusitania in 1915; about twelve hundred civilians died—including over one hundred Americans. President Woodrow Wilson demanded that Germany stop submarine warfare, and Germany acquiesced for two years.
Second, Wilson also wanted to "make the world safe for democracy." His idealism helped take America into the war. He thought a "just peace" would end war forever.
American society was affected in at least three ways by the war. First, the labor force changed as black citizens and women began to do jobs that had previously been done only by white men. Southern African Americans moved North by the thousands for these jobs. Women became active in all professions. Second, there were race riots caused by changes due to the war: whites citizens did not welcome the influx of black citizens to their communities. In 1917, there was a bloody riot in East St. Louis. Third, civil liberties were suppressed during the war. Thousands of people, including the famous Eugene Debs, were jailed.