It looks as if so far, I can take the cake for the most useless language "larned" in high school: Latin. Yes folks, the dead language.
Realistically it was reasonably useful, as it's strengthened my grasp on English vocabulary, and made me realize just how much the rules of a language are stretched when you listen to music or read the poetry. (That was a nightmare, Latin poetry... <shudder>)
That said, immersion in the language and culture is probably one of the best ways to learn it.
Another tip is to learn a language that's similar to English, as it would be much easier. Spanish, French, Italian, or to a lesser extent, German or Russian are good choices. Most of the Asian languages or the languages of the smaller island nations (Hawiian, etc.) would be hellaciously difficult to learn, because the evolution of the language had no common roots to English. Hell, even a few of the Asian languages evolved completely differently from each other.