I assume you are referring to boarding the monorail in DTD. If so, there is a park entrance turnstile at the monorail. If you are boarding before 11 am, you must have a Disneyland reservation. They will scan your ticket as you enter. If you are boarding after 11 am, you can have either a Disneyland reservation or a DCA park hopper reservation.
You can scan your ticket at the Monorail entrance and ride it into the park from DTD. I've done this many times when staying at DLH. But, as LKing said, you need a a Disneyland park reservation for that day if you're trying to go for opening.