Please explain the Battle of Shiloh simply?

I have trouble understanding the material about Civil War battles that my history teacher gives us (It's very complex high school/college level material, and I'm in middle school.) and I have to write a paper on the Battle of Shiloh but I don't entirely understand what happened. It's due Monday and all I really know is that Grant was waiting at Pittsburg Landing for reinforcements and Johnston attacked, the Confederacy nearly won on the first day but the Union defeated them on the second, and that there were insanely high casualties on both sides. Please answer in your own words, easy to understand, don't just copy and paste stuff. Help!