"But the day of the Lord will come like a thief. The heavens will disappear with a roar; the elements will be destroyed by fire, and the earth and everything in it will be laid bare." NIV translation
Because most people have stopped expecting Jesus to return at any moment, His return will be unexpected like a thief breaking into your home. When He returns, the delay will be over and judgment will be swift. The old things will perish in preparation for the unveiling of the new.
"Since everything will be destroyed in this way, what kind of people ought you to be? You ought to live holy and godly lives as you look forward to the day of God and speed its coming. That day will bring about the destruction of the heavens by fire, and the elements will melt in the heat." NIV translation
Because we know these things, our lives should reflect the fact that these things are only temporary. We should focus on the things that are lasting. The "speed its coming" part has caused much confusion and even led some people to do some very strange things. It simply means that we tell others about Jesus and how they can be saved from the coming wrath.
God knows all that are His and, when the last one is saved, He will send Jesus to collect His sheep. We speed this process by being effective witnesses which is an important purpose of this website.
"But in keeping with his promise we are looking forward to a new heaven and a new earth, the home of righteousness." NIV translation
This old world is passing away soon but the good news is that there is a new one coming! Revelation 20 describes this new heaven and earth that we inherit through the blood of Jesus Christ. Many people think that when we die we simply go to heaven, hang out on a cloud, and play a harp but that is not so. We are going to be on this new earth and it is going to be without the effects of sin.
"So then, dear friends, since you are looking forward to this, make every effort to be found spotless, blameless and at peace with him." NIV translation
If you are looking forward to living with God in the new heaven/earth, which is the home of righteousness, you will not want to be a party to sin. It is interesting to note that Peter assumed that those who believe in Jesus were looking forward to it. We all have to ask ourselves whether we are truly looking forward to this or are we happy in this world.