Your ASL dictionary will get you vocabulary to practice, but it does not teach you why the shapes, placements, and movements are significant (where they come from).
Also, ASL dictionaries are usually written with English entries showing ASL words. If you actually see a video of someone using sign language, there isn't a way to figure out what those words mean unless you've already memorized a lot of words.
Just like anything in life, you have to live it to really know it. Find someone to learn with you. Without a reason to practice, you may loose interest.