One way into a career as an ASL interpreter is to join RID (Registry of Interpreters for the Deaf). You are usually expected to have a bachelor's degree or an equivalent level of education, and in addition to take exams leading to certification as an ASL interpreter. The best known is the NIC (National Interpreter Certification). This has an exam, as well as an interview process and a performance evauation program.

You can find some useful information here on the type of work that you are likely to get, and the length of time it usually takes to qualify. If you scroll down the same page you will also find links to some sample test papers, which will give you an idea of the standards involved. There is also information on how to join as an associate, which may be helpful if you have interpreting experience but don't qualify as a member. You can also use the website's contact details to get in touch with RID members directly.