How did yellow journalism impact America and make it join the war?

Although you do not specify which war you are talking about, I assume that your question has to do with the Spanish-American War.  That war is the only war where yellow journalism is cited as a major cause of American participation.  Yellow journalism helped cause the US to enter this war because it made Americans hate Spain and it made them believe that Spain had committed an atrocity against the US.

Yellow journalism was characterized by sensationalism.  These journalists were more interested in getting exciting stories that people would read than in publishing the truth.  Because of this, they ended up printing many stories that were exaggerations, unfounded conclusions, or just plain falsehoods. 

Before the Spanish-American War, there had been a rebellion in Cuba against Spain.  Spain had, of course, been trying to quell the rebellion and keep Cuba as a colony.  Many Americans were opposed to this because they did not like the idea of European countries having colonies in the Americas.  Because Americans supported the Cuban rebels and opposed Spain, yellow journalists printed many stories portraying the Spanish as brutal and cruel towards the Cubans.  These stories predisposed Americans to dislike Spain.

Ultimately, an act of yellow journalism helped to directly bring the US into the Spanish-American War.  The US government had sent the USS Maine to Cuba to help protect American interests.  While it was in the harbor at Havana, the Maine blew up with much loss of life.  Yellow journalists published stories asserting that there was proof that Spanish agents had blown the ship up.  Americans believed these stories and were therefore in favor of war with Spain.

Thus, yellow journalism made Americans more likely to hate Spain because of Spain’s occupation of Cuba.  It then convinced Americans that Spain had sunk the Maine, thus causing the US to go to war with Spain.