|Hint... - Italian is quite a complex language for its grammar and for the huge amount of irregularities. I think the knowledge of a Romance language like French or Spanish might prove beneficial as italian syntax is rather similar to that of those languages. In this case, making parallels between the known language and italian can really help. If you are new to Romance languages, I think before moving on to other issues, it would be good to learn about the structure of the language and some of its mechanisms and patterns, thus building a solid base which u can use to build up superior language skills.
The thing becomes incredibly more complex when it's about slangs. Unlikely many other countries, where slangs are made of some intonations and just a few different expression, Italian has a true galaxy of slangs which are so different from one another that it is not infrequent that 2 italians speaking strict slangs don't understand each other. So, before starting the study of a slang (which is in my opinion a real fascinating adventure) I reccommend a good knowlegde of "official" italian.
Hope you did not fall asleep reading this....