There's a really good book by the tango pianist and composer Horacio Salgan, but I don't know if is available outside Argentina. Anyway... forget about percussion (specially castanets) if you are suposed to write something close to a real argentinian tango -tapping at the instruments is more idiomatic-. I should mention that the examples shown by Oliver are more related to a "tango a la Valentino" (maybe that's what you need). Check out the sheet music in that link posted by Julio. Think of lots of rubato. And at least some phrase should be in a minor key (not necessary but...). For the basic instrumentation use bandoneon and piano. Then, if you like, add a string chamber orchestra, or violin, guitar and bass (I'm thinking of the Piazzola quintet). Maybe a flute (the oldest type of tango used just flute and guitars). Perhaps some clarinet. Definitively no brass. I'm not an expert in tango, just an argentinian.
Best regards (and sorry for my poor english)