I've been a special education teacher for 14 years - most of that in a self contained classroom with kids w/ autism. As some have said, each child is different and has different motivators. Talking with the parents before hand will be a great help.
Things that have worked for many of the children I've worked with:
A visual schedule - this is something that can be on the corner of their desk so that they can see what is coming up and when breaks/choice activities are coming.
Frequent breaks - long periods of work can be tough for any student, but even more for a student with autism. Sometimes I even have to break up assignments into chunks and make separate sheets. If they get a worksheet with a bunch problems on it, even though they only have to do the first row, it can be overwhelming. Make a copy of just the first row, then a sperate copy with just the second row that they can do later. Hope that makes sense.
As others have said, a separate workstation away from distractions - it depends on the student, but many need a place away from distractions and other sensory distractions. I've had some kids that had major issues with fluorescent lights - things that you or I don't even notice. I ended up just buying lamps for the room and all was well.
The parents will give you a better idea on sensory tools to use - the most common that I've found helpful are bean bag chairs, weighted blankets, swings, sandbox, and tupperware filled with different items (rice, marbles, play-doh) that they can run their hands through.
Computer/iPad time as a reward - letting them earn time on the computer or iPad is a great motivator for many kids. I've worked at schools that don't allow
YouTube, but working with the IT department we've figured out ways to allow them to watch short clips of things they are interested in. Big motivator for some.
Those are just some things that jump out at me - the parents will be your greatest resource. You also should Think about setting up a time where the student/parents can come in and see the room, meet you before all the other students come. That can be a big help for that first week.
Hope this helps! Feel free to ask away if you have any other questions!