When does the movie Wonder actually start with the time of 5:20pm?

Ten minutes of previews begin at the movies advertised start time.

Is this your first movie?

...actually, AMC got themselves a twenty (20) minute pre-show of trailers and other ads at the posted start time of each moving picture.

The commercials and three or four previews take at least 15 minutes from advertised show start time.

I would say 10 mins of previews at the least.

Check the show times. The updated information for the specific day is available online.