How can we practically base our decisions on scriptures when the scriptures are so vast?

From: Subir Das

You have mentioned in your “Daily Thought dated 9.11.2011” that  “This indeed is the reason why the Lord urges us in the Gita (16.24) to forever make the scriptures as the first and the last basis of our decision-making”. But given the fact that our scriptures are so vast (I hardly know about the scriptures), how can common man like me arrive at a decision on the basis of scriptures? What to do in such a situation? Please guide.