What does it mean to be "ready" when Christ returns in power and glory?

For Believers it means fulfillment of the promise of redemption into an everlasting life with the Father, Son, and Holy Spirit, where there is no weeping, no worry, no death.

If you are not ready and want to be ready, you can take this very simple step. Invite God, through the Person of Jesus Christ who sits at His right hand, and who has paid the price for your sins, to come into your heart.
In the Book of Luke, chapter 11, verse 9, Jesus said, "So I say to you, Ask, and it will be given you; search, and you will find; knock, and the door will be opened for you."
He said the same thing in other places as well. It's a promise to YOU. Ask HIM as He said to do. I can put the words into your mouth for you or you can make up your own. It doesn't matter. What matters is that you choose voluntarily to surrender control of your sinful and uncertain existence to the One who can SAVE you. If you pray the prayer of salvation and mean it in your heart, you will become a new creation, born again into the family of God. He will not turn you away. Perhaps these words will suit you:

"Lord Jesus Christ, I know I am a sinner and cannot save myself. I don't know where else to turn. If you truly exist Lord, and are truly my Savior, I ask you to come into my heart and give me eternal life. This is not a challenge Lord; it is an invitation from a human being who is not sure of anything. I want to be sure. I want to KNOW Lord. Please come and show me. Amen"

If you are a sincere seeker after the truth, you will find that the truth you seek will be revealed to you in no uncertain terms once you have truly asked God to reveal Himself to you and take charge of your life. You have to personally surrender to His will and His grace for this to happen for you. You can do this any place or any time and under any circumstances. But it must come from your heart. You can't fool God. If you could, then He wouldn't really satisfy the definition of God, would He? If there IS a God who cares for you, who better to trust with your life? Once you have submitted yourself and invited God in, He will lead you to the people and places where His grace toward you will be manifested so you can begin the joyful journey of walking in His light. Thanks be to God.