Starting your AMCAS
When filling out AMCAS, you will need two transcripts. One to use for entering your individual grades onto the AMCAS application, prior to submission. The other will have to be an official transcript sent to AMCAS, which they will use to verify your grades prior to sending it out to schools. This verification process adds time to when your application will reach schools.
Therefore, the later you apply, the more backed up AMCAS grade verification will be. Thus, it will delay your application from reaching the school. As simple as that.
Next, a list that has all the activities that you've been involved with will now come in handy. You can only list things that you've done since beginning college. Nothing from before then will count (i.e. anything in high school).
I think you are able to list 10-15 things and write about 250 words on each. So make sure you have several clubs, honors, jobs, positions, awards, etc. to write about. But remember, quality over quantity. Interview questions will stem from these things.