1st what state are you in? It varies from state to state. Many states don't required liscence, but certification (many are working on it to be sure every state requires liscense).
There is also state and national levels.
In Michigan it is called QA (quality assurance) Levels I II and III. You need level II to interepet most places, III or above for some cases (Mental health, Doctor appointments, courts ect....)
Here is a web site with some info on it.
http://www.deaflinx.com/terp.html
This is the NAD site, a good place to start
http://www.nad.org/site/pp.asp?c=foINKQMBF&b=91587
RID is Registry of Interpreters for the Deaf
http://www.rid.org/
http://rid.org/119.pdf
As mousermerf said you are going to need much more then a basic course. I have been signing for 15 years now and I perfer to sign for myself (I am a therapist that works with deaf and hard of hearing people) then be an interpreter. It is very challenging work.
I went to Madonna Universtiy in Liviona Michigan and have a bacholors degree in sign lanuage services (along with social work, and a master in social work also.)
It takes as much work to be fluent in ASL as any lanuage. While I learned a great deal in school I really only started to be fluent once I started working in the community and useing it daily.
Good luck. It can be a great field to get into and the pay can be good too (around 25$ a hour). It can be hard to find consistant work 40 hours a week though. I have a friend who interepts on cruise ships often. She loves that. :sohappy: However it will require as much work and training as any college degree
There is a lot of web information out there (Deaf people LOVE the internet. You don't need to hear for it!)
Good luck.