Logically, the first step to prove a resolution is to understand it. Debaters open their cases by clarifying the advocacy of both sides. 

Definitions matter: but they're an expenditure of your time. Defining every word will drain you. At the same time, ignoring them entirely can be dangerous. Here’s how you decide when to whip out the dictionary:
 

Will the opposing definition be reasonable?

Some words have really obvious meaning. No one's guessing about what the words “United States” mean. “Transportation Policy,” meanwhile, could bear some clarification. If the meaning is so obvious that any deviation from the man-on-the-street interpretation feels strange, you don’t need a definition. Normally, the word “should” falls into this category as well. We're not running definitions for super-obvious words: in the worst case scenario that our opponent disagrees, he's not going to win. 

Disagreement over words like “is,” or “resolved” is blatantly petty; it is very hard to sell to a judge. Definitions can say crazy things, for example: “The United Nations is the name of a Slovenian punk band from the 1970s.” If your opponent runs a definition like that, the judge will be very sympathetic to your reasonable definition presented at the first opportunity (2AC or 1AR).
 

Will the opposing definition threaten your case?

If your case is so incredibly topical that any reasonable definition accommodates it, you probably don’t need a definition. For example, if your Stoa policy case raises the interstate speed limit from 70 to 80, you don’t need to define your way into “transportation policy.” There may be other definitions, but they can’t hurt you.

In other scenarios, your case will depend on a certain range of definitions to work. For example, most Stoa LDers need “preemptive warfare” to be defined as an attack made in anticipation of some counter-threat. Many NCFCA policy debaters need reform to be defined as a mere change rather than a complete overhaul.

If you choose a standard definition, or one that favors your opponent, there likely won't be disagreement. On the other hand, if the definition is what permits your advocacy – or even builds it – you should be ready to defend it. If your case is incredibly dependent on the definition, consider running subpoints within the definition statement explaining why it’s a good definition – for example, because the dictionary source is so credible. You're happy to take the extra effort for your definition: a counter-definition could lose us the round.
 

Run a definition because it's worth your time.

If disagreement on a term's meaning is reasonable and dangerous, you should define the term in your first speech. If not, you can probably get away with spending your time elsewhere. 

In some resolutions, you'll offer as many as 4 definitions. In others, you'll skip definitions entirely.

Debaters usually define out of paranoia: "better just define a bunch of these words to be safe!" But remember: every second is precious. Don’t run definitions just because they’re in the resolution or because a counter-definition exists. Run a definition because it’s worth your time: it gives you an advantage by making your case stronger and clarifying the link between your advocacy and the resolution.