In the real world, peer pressure means “the suggestion to do what we don't want to do so we will belong.”
That desire to belong is a huge motivator and can move us to act against our will, whether we're being pressured by a member of our family or of a street gang. The best defense against peer pressure is understanding who you are and why you are here.
You are a child of God and you already belong to Him (1 Corinthians 6:19-20, 7:23). As His child, God has a plan for your life, one He guarantees will bring you peace and joy (Jeremiah 29:11). Believe in this hope!
Nobody else sees the big picture like God, so the smart thing is to do His will. If you follow Him and do what He says, your life will be one of peace and joy. If you do not, and instead give in to peer pressure, you could really mess up your future and your life will be one of fear and pain.
Many a person has gone astray from God's will for them by giving in to peer pressure. While not all consequences are terrible, there are always consequences. And while you can recover from mistakes, it's easier to not make them in the first place. This one thing, if taken to heart, would see you through every difficult situation, because you can ask yourself, “Is this something God would want me to do with the life He gave me?”
Educate yourself right now, before the pressure comes. Speak with others who have given in to peer pressure, especially as it involves drugs, alcohol, and sex. While you might not think drinking alcohol is bad, or taking mind-altering drugs is harmful, or that pregnancy couldn't happen to you, there are plenty of horror stories out there that will convince you otherwise. Do your research. Truly, knowing is half the battle.
There are people around you who don't require you to act against your will to belong. Hopefully you have a loving family or group of friends who accept you as you are. Even if you don't have loving family or friends, you can find a church filled with people who will accept you. A good church can be a great family.
So if you are ever in a situation where you feel pressure to go against God's will for you, ask yourself why you are tempted to conform. Is it because you want someone or some group to accept you? Is it because you have a sinful desire you want to fulfill, and this seems like an easy excuse? Regardless, ask God to give you strength to escape the situation, even if it means losing face or a friend.
And remember, the amazing thing is that God's whole purpose in life is to get you to accept Him!