Hi. I need to learn as much as I can about autism. My cousin is autistic and I want to be better able to understand him. 
Hi there. Autism is an unfortunate birth defect that is commonly associated with troubled social interaction and communication.
Are there different kind of autism? And are there differing degrees of severity?
There is. One of the more known ones is Asperger Syndrome. Aspergers makes it almost impossible to interact whatsoever with anyone. 
My cousin interacts fairly well, but he struggles with it. How common is autism in the general population?
I am not 100% sure on how common it is, but to answer your previous question yes there are differing degrees of it.
I think it seems like autistic people often have gifts or special talents in specific areas, too, like my cousin is excellent at music.
They do seem to have unique talents. It is interesting how autism even develops and sad at the same time.
I know autistic people have a hard time in school and often have to take special classes.
They do, but with the right medications they can function and succeed just like everyone else.