When you want to look up an expression, there are two possibilities: you might find it in the entry for the first word in the expression, but more likely it will be listed in the entry of the most important word in the expression. For example, the expression du coup (as a result) is listed under coup rather than du.
Sometimes when there are two important words in an expression, the entry for one will cross-reference the other. In looking up the expression tomber dans les pommes in my Collins-Robert French Dictionary program, I began my search in the tomber entry, where I found a hyperlink to pomme. When I clicked over to the pomme entry, I found my expression translated as "to faint/pass out."
The important word is usually a noun or verb - pick a few expressions and look up the different words to get a feel for how your dictionary tends to list them.