A critical essay is a writing form that is intended to analyze, interpret, or evaluate a specific text. Based on the question, it appears that you have to critically write about Bradford's Of Plymouth Plantation. This might seem like a daunting task, but all that you are being asked to do is make some kind of claim or argument about ideas, themes, characters, or other aspects that are in the text. Once your claim is made, the essay's job is to then support the claim with evidence from the text. My recommendation is to analyze a theme or character, because those things have a nice interplay between them. Themes are typically supported with characters, so it is fairly easy to discuss both. This particular piece has themes about faith, Christianity, government styles, and racism. If analyzing a theme seems too daunting, then break down a character. Tell your reader about Bradford, Standish, or Squanto. Make an argument about their actions and support that with quotes from the text.