1 Kings 22:1-28 also 2 Chronicles 18:1-27

What is this passage all about?
Ahab asked Jehoshaphat to accompany him to war and Jehoshaphat told him to seek the counsel of the LORD through a prophet, who foretold doom for Ahab, contrary to the other pagan prophets.

What can I learn from it?
The parallel passages in 1 Kings and 2 Chronicles which tell of this event are almost word-for-word accounts. 2 Chronicles adds one detail which explains the willingness of Jehoshaphat, king of Judah to visit Ahab, king of Israel. Jehoshaphat had married into Ahab's family to form an alliance. So in travelling to Samaria he was visiting an in-law.
Despite Jehoshaphat's obviously questionable decision-making abilities (as evidenced by the afore-mentioned marriage), he knew enough to seek for advice from a prophet before going into battle. At first he seemed willing (v5), "But Jehoshaphat also said to the king of Israel, 'First seek the cousel of the LORD.' " When Ahab produced about 400 prophets who all encouraged them to go to war, Jehoshaphat still held back. He wanted to hear from a prophet of the LORD, not just any old pagan prophet (v7). "But Jehoshaphat asked, 'Is there not a prophet of the LORD here whom we can inquire of?' " Ahab's comment was that the only prophet of the LORD who was available was Micaiah, who always prophesied bad things. (Obviously Ahab never listened to Micaiah, otherwise he might have realised there was a reason for the LORD always sending bad news.) Jehoshaphat rebuked Ahab (v8b): " 'The king should not say that.' "
After Ahab warned Micaiah to tell nothing but the truth, Micaiah prophesied doom for Ahab and the scattering of Israel, who would be (v17) "like sheep without a shepherd". Micaiah also explained why the other prophets were encouraging Ahab and he wasn't. The LORD had allowed a lying spirit to be put into the prophets' mouths to entice Ahab to his death attacking Ramoth Gilead. Not liking what he heard, Ahab sent Micaiah off to prison until his safe return (which wouldn't, of course, occur).
It does help to be reminded that it is possible to have stuffed up royally with some decisions, going completely against the will of God, but to still be able to return to a state of relative wisdom where one can see the importance of doing things God's way. Jehoshaphat had done wrong in allying himself with the evil Ahab through marriage, but at least in this episode he spoke wise words into Ahab's ear. Ahab couldn't say he hadn't heard the truth about what would happen to him, because Jehoshaphat and Micaiah had made him listen.

How does this help me to worship God?
I can always come back to God, especially when I feel the burden that sin has been on me, separating me from God, and ask Him for forgivenness. Then, I can go on, through His grace, forward in our relationship. Once again I can serve Him, and in doing so serve others.