Chris,
As is the case with most ballet, it's all about recreating the original. There should already be run sheets from the last time it was performed, as well as an archival tape. When I get a new assistant on a show, I try to spend part of their first day with them watching the video once, then watching it a second time with the run sheet. (Very similar to how you would look at a script - read it once to get familiar, read it a second time and start taking notes.)
If it's a well-known company, odds are they have a headshot page or series of clickable bio/headshot pages on their website. I've had assistants create flash cards to help them learn the dancer's faces and names. That will be very helpful to you, and will make it easier for the SM and you to communicate about the dancers.
Anything else that would be specific to getting you prepared is most likely a question for that company's SM. Every company is different - some have large admin/support staffs, while some don't. It may be a good idea to find out what all of your duties may be, or if you're strictly the ASM and nothing more. Ask if you will be responsible for wrangling children/students in the production or if someone else will take care of that, if there are non-dancer guest walk-on roles - will you be responsible for getting them into the building and onstage, do you need to be available to count in Supernumeraries for their entrances? (my ASM has a ton of other activities, like casting Supernumeraries, securing parking for volunteers, sending student Mice into a scene, etc).
Also be sure to find out if you're working in an IATSE house or not. If you are not, then you may be heavily involved in scene shifts, if you are - then your goal will most likely be to make sure paths are clear of dancers for the crew to perform shifts. This information should be laid out on the deck/run sheets, but it's best to ask.
See if you can set up a meeting with the PSM/SM soon to go over what is exactly expected.