Were do I find out which definite article goes in front of a word?
la, el, los, las, and their like all mean "the" but which one is correct depends on what I don't know?
Here's an article from the Reference section on definite articles --- It's called "Why do you put "el" and "la" in front of all the nouns?"
Actually Lesson 1:3 of the Spanish course on this site will tell you all you need to know. There are some words that are trickier, but there are some general rules that you can follow for even these words.