As ShootingStar noted, there's two important things about learning a new language, first vocabulary, then grammar. And grammar can vary widely from language to language. I studied French in high school, came out first in the class, but some years later I found out the hard way that there's a big difference in the way French was taught in the Mississippi Delta, and the way it was rapidly and natively spoken by real live French folks ;-)

I haven't kept up with my French since then, not even un peu... but in college I was required to take a year of German. (back then you couldn't get a chemistry degree without being able to read the Beilstein and Gmelin handbooks and the old German articles in chem lit.) This time the professor focused more on learning the grammar than rapidly building vocabulary, and it went a lot easier, and I retained more of it a couple of years later when Uncle Sam sent me to the U.S. Army in Europe for a six-year immersion course. After nearly 20 years I can still pretty well hold up my end of a converstion auf Deutsch, and it's a lot of fun listening to the dialogue in soem of the old war movies from the 60s and 70s and chuckling at what the folks writing subtitles left out ;-)