I noticed that you posted this to /r/HomeworkHelp as well - and I'm afraid that that's where this question belongs. We are not here to do your homework for you, I'm afraid - and we can't tell you how to write your essays. If you need further help, assuming that you haven't left it until the last minute, I strongly recommend just asking your teacher :)
Best of luck!
Well, first place to look is the FAQ section on the start of World War I. That should give you a little bit of a clue as to what led to the war itself, which is good for background information.
Now as for the United States' entry into World War I, there's a lot of factors. I won't give you a bunch of information because I feel like having you learn to research and find the proof yourself is far more important for your education, but purely a few ideas that you should research:
Look into the Lusitania incident.
Look into unrestricted submarine warfare by Germany on the whole.
Look into the Zimmerman Telegram.
Look into competing propaganda put out by German-Americans and those who supported the Entente powers.
Those should give you plenty of background on why the US entered the war. Again, I don't want to make it too easy, but each of those played a crucial immediate part in the entry into the war. There's more subtle things (in my eyes) like business interests, general goodwill towards the Entente powers, etc., but those are the most important in the immediate case, I'd say, and would make for the best writing.