The US did join World War 1, but they joined late. Some of the reasons they did include:
Zimmerman's Telegram - a telegram intercepted by a british officer, sent by the germans to the mexicans saying that if mexico helped germany in the war by fighting the US, then germany would guarantee that mexico got land back from the US they lost in a previous war.
Lusitania - Lusitania was a british passenger ship sailing in the Atlantic. At the time, german submarines were firing at all unfriendly ships because they might be caring weapons for the other side. Americans died when the germans sunk the Lusitania, which upset Americans back home. (However some research says that the Lusitania was in fact caring weapons for the British from America so the Germans did have the right to sink that ship)
America felt closer to the side of the Allies (Triple Entente) rather than the Central Powers (Triple Alliance) because they spoke the same language as the British and had ties historically with them, which is why they joined Britain and France in World War 1.
The reason that Woodrow Wilson, the president at the time, decided to push for war was to "protect democratic principles" because his aim was world peace. (Search Wilson's fourteen points for more information on that)
However if your essay is a persuasive essay on why the US should have joined World War I, that's more opinion and I'm not sure how to argue that.
