1. It's not easy as some think it is.
Some people jump in high aspirations and have no real training in using ASL in their everyday things. You must go through school. We recommend at least a bachelors degree minimum. Many people call me up and ask me to do a program for their kids or at schools and these students (children and adults) always think that if they pass my beginning Sign class, they're qualified to work as an interpreter or something using ASL.
2. ASL is not universal.
ASL comes from French Sign Language (FSL) except many assume it's from British Sign Language, since the spoken language there is often English. Truth is most countries have their own sign language, which is full and beautiful and a bona fide language there. Mexican Sign Language, French Sign Language, Italian Sign Language, all legit languages. American Sign Language is used in North America and some of Canada (and sometimes a little of Mexico). And even within America, there are accents and variations. Words such as EARLY, BIRTHDAY, PIZZA, OUTSIDE, etc., are signed differently in West Michigan than, say, Texas. It's all different.
3. Facial expressions are grammatically necessary in ASL.
If you're not making facial expressions as you sign ASL, you're not signing the true language of ASL. There are necessary aspects of signing ASL that involve sticking out your tongue. blowing, lifting or lowering your eyebrows, etc. I've seen some great signing from high school music groups who try to play their music and sign ASL at the same time, but they don't have any expressions. Signing ASL has requirements--especially facially--and signers need to know that. There's more information on my web site about mouth morphemes and other such things. Check it out at firstname.lastname@example.org.
4. English Sign Language (ESL) is not the same as American Sign Language.
There are still many people who don't know the lingo. ESL is an English sign system created to help children to learn how to sign and read at the same time. However, ESL isn't a real language and I don't recommend learning that. It can be a bad experience and get you hooked on it. It's a signing system (not language) where students learn to sign every word, every part of a word, exactly as they'd speak in English. It's confusing and it takes a lot longer to express yourself. ASL has a life all its own.
5. Certification.You will not know enough the first year or two to test for certification. No, I don't mean the first set of classes, I mean "year." It takes about seven years to become fluent in a new language. ASL is the same. You will require lots of practice, attending classes and workshops, reading, etc. To take the test to make you a certified interpreter, you'll have a written test and a signing test (both expressive--you sign what the hearing person is saying, and receptive--you speak what the Deaf person is signing) and they're quite difficult. There is state testing and then, if you want, there's national certification (which is the best option for someone wanting to interpret).