Review v1-5 first, then move on to v6.
6 But when he asks, he must believe and not doubt, because he who doubts is like a wave of the sea, blown and tossed by the wind. (NIV)
6 But let him ask in faith, with no doubting, for the one who doubts is like a wave of the sea that is driven and tossed by the wind. (ESV)
When we ask for the wisdom in v5 we must pray with absolute trust in our sovereign God, without doubt both mentally and morally. God does not lie. God does not break any promises. God has promised us wisdom when we ask, and if we ask (with right motives and seeking His glory), He will answer.
Repeat this verse over and over today.
LeRoy Eims on the importance of Scripture memory: "I think two of the master strokes of the devil have been to convince people he doesn't exist, and that Scripture memory is only for children. He remembers the humility of defeat when Jesus Christ, who was tempted in all points like as we are, met him with the Word of God." (What Every Christian Should Know About Growing; p. 26-27)