Scholars and sign language speakers alike consider International Sign Language (IS) the sign language equivalent of Esperanto. Also called Gestuno, IS is a constructed language with over 1,000 signs intended to unify sign language speakers around the world. Since it's primarily a lexicon with no distinct syntax, speakers of IS tend to sign it using the syntax from their native sign language. There are no native speakers of IS, so you'll likely have to use a dictionary to learn it.
Check out an International Sign Language dictionary at your local library (or, see Resource 1). This will be in video or photographic form to help you visualize the signs. Check out a book on American Sign Language as well (or, see Resource 2).
Choose one hand to be your dominant hand, the primary hand with which you sign. Typically, this will be the hand you write with.
Practice forming the signs in the IS dictionary. Shape and move your hands exactly as they do in the dictionary, as this is similar to pronouncing words properly aloud.
Practice forming sentences in IS. Use the book on ASL for instructions on syntax, since IS doesn't have its own syntax.
Find at least one person in your area with whom you can speak IS. Contact a local center for the deaf and hard of hearing to find out if anyone there can help you. The best way to become fluent in a language is to use it regularly.