Hi!
I'd recommend reaching out to your University's Disability Department, they are in charge of arranging accommodations for students with injuries, disabilities, and injuries that impact their school life. In my experience, they require medical verification of your disability (ie medical records) which usually means you have to fax a Release of Information form to your medical provider(s) and ask them to send the requisite documents to the school. Expect a 2-4 week waiting period depending on the size of your provider's office/hospital or their waiting period.
As for what to ask for? That all depends on your symptoms. I usually plan for the worst and ask for flexible deadlines, extra time on exams, more time to drop classes and ask the Disability Office to let my Professors know that I may have a spotty attendance, all of this then goes in my file and is automatically sent to my Professors. I'd also recommend taking advantage of any online classes your University might offer, most of it is textbook work so you get to avoid the light of screens, and get to work around your migraines instead of sitting in a lecture hall.
Hope this helps, good luck!