I agree with everyone else's comments on don't use Rosetta Stone, the Romance languages will be similar, etc.

Studying it for 4-6 years could get you very far, or not far at all. It depends entirely on you. From my own experience with school language classes, I would say that most of the class is probably going to not study except for quizzes and tests, and will not practice. They'll do enough to get good grades, but won't actually be able to use the language.

So if you want to be able to use the language, study it on your own alongside school. Learn vocabulary on your own, learn some grammar ahead of time, and practice writing and making sentences with the new vocabulary and grammar. Also, practice with your teacher. It would be good to get an online language partner down the line, but when you're just starting out, your teacher is probably the best. They know which words you already know for sure, so they'll try to limit themselves those in conversation, and you can use some of the new words you've learned. Good luck!