Check out examples at sites like You Tube by searching "fake commercials". Study as many examples as possible. Watch them first for fun, then watch the best ones multiple times, paying attention to how they are put together. This includes action, timing, characters, music, dialogue, and all other aspects you wouldn't generally think about when watching purely for entertainment purposes. Make lots of notes.
Review the articles below in order to get ideas for how to write your script and learn how to follow a correct script writing format. Take notes on at least 5 of these articles using the Cornell Notesheets.
Decide on a product you are going to produce an ad for and write a rough draft of your script.