Polya -- found in translation

Whenever my students complained about word problems, my standard response was "life is word problems. No boss is ever going to walk up and ask you to solve this quadratic equation." I've logged a lot of cubicle hours since then and the observation still holds.

As for the actual mechanics of actually translating prose into algebraic expressions, you will find few explanations better than this one from George Polya's How To Solve It. Pay particular attention to the distinction between word-for-word translation and idiomatic translation.