J.P.
First, be proud that you are willing to get help for your child. I have a daughter diagnosed recently on the autism spectrum, and just this year has required treatment for generalized anxiety disorder. I would suggest using your pediatrician as a resource. We found that cognitive behavioral therapy is helping her, but it's a long road.
There are SO many books out there, I also suggest read, read, read. There is also a useful website I found, www.worrywisekids.org. I am sure there are many more!
Good luck to you. The hardest part for me was, and still is, asking for help. But it will be worth it.