Hey there!
I'm an OT. I've worked with kids/adult with autism, though that isn't my focus at the moment.
The OT will probably ask you questions about what issues are difficult for you, and ask why you're there to see them. (if they don't already know)
Depending on what you're seeing the OT for, they might have you fill out some paper evaluations - if you're there for sensory processing problems, they will probably have you fill out a paper evaluation asking about different sensory issues, etc. If you have problems with writing, they might give you a writing evaluation. They might test your gross motor skills, or ask about your daily living skills or social skills, or whatever your most pressing issues are. Then you set goals based on your evaluations and come up with a plan to meet those goals.
So basically, the OT will get your subjective opinions about what you need, will objectively evaluate you using some kind of evaluation tool, and then make a plan and set goals. In further sections you'll work on those goals.
OT for autism is generally pleasant and fun, like Trinket said, but what you'll do depends on your age, level of function, and other personal things. It is usually very much based on what you want and need, and your relationship with your OT is more like a team than it is like a doctor-patient relationship. Hope that helps.