Ouch, Argentina...
An argentinian here, currently living in Spain and polyglot of 5 languages. Well, first, it will be complicated because argentinians speaks with a lot of slang, but in front of you they will not.
The best way is by try/error. I started to speak italian in 15 days doing this, knowing that my mother language is a romance one, like Spanish, learning a new one was piece of cake, but coming from an american, things can be a little different, specially when pronouncing the vocals and some other letters but with the time and able to hear, I beat you can even speak some of the slang.
I've meet many americans in Buenos Aires and so far I've remember, most of them learnt a good spanish base with slang. Funny, but they loved.
If you go to Spain, you will find the spanish language isn't so singed as the argentinian, also the spaniards doesn't have so much slang so the language is plain boring.