When I was in San Diego, I asked to a girl "how can I get to the freeway?" She answered me, "Go straight on, you can't fuck it up." What does it mean? Is this a usable phrase or it is too vulgar? Is this american slang?
She meant that her directions were so simple you couldn't misunderstand them. To fuck something up means to get it very badly wrong (see FUBAR).
It's definitely not for use in formal situations, or for use around those with sensitive ears.
It's also not exclusively an Americanism, as I've heard it used in the UK and other English language countries, although I couldn't speculate as to the origin.