If I was a lookout when my friend took a Halloween decoration off someone's lawn, will I go to jail?

You have been charged with a "misdemeanor", which is a crime less than a felony. Most misdemeanors allow the Court to include less than 1 year of jail time when sentencing a defendant, after they plead guilty or are convicted of the crime by a judge or jury. The level of maliciousness (from a judge's perspective) and a prior record can lead a judge to issue such jail time. However, assuming a typical Halloween decoration without a prior conviction, an experienced attorney would likely be able to convince a prosecutor to reduce the charge and waive any jail time, or convince a judge not to issue jail time as part of the sentence. Let me know if you have any other questions.

It is impossible to predict what type of sentence you could receive. Likewise, it is unclear exactly what you are being charged with. The sentence will likely depend on exactly what you are being charged with and what your criminal history looks like. As a misdemeanor, you may be able to avoid jail and just have to pay a fine, but without knowing the exact charge and your history, it is impossible to predict.

You are looking at a criminal record which could follow you for the rest of your life, get an attorney now!

Up to 93 days in jail. Hire local counsel asap.

Yes, you can be charged as an accessory. That puts you on the hook just the same as the principal actor.