One reason I wanted to reprint your letter is because it vividly illustrates an important spiritual truth: Stay away from people who you know will tempt you to do wrong. The Bible rightly says, "Do not be misled: 'Bad company corrupts good character'" (1 Corinthians 15:33).
What can you do? First, realize that God doesn't want you to destroy yourself with drugs (or in any other way). God loves you - and because He loves you, He wants what is best for you. He wants to put your feet on a different path - and He will, as you turn your life over to Him and learn to walk with Christ every day. This is why the most important step you can take is to turn to Jesus Christ and open your heart and life to Him. You need His help - and He wants to give it to you.
Then deliberately turn your back on your old life and discover the new life Christ has for you. Avoid people and places that will tempt you to use drugs. Then seek out a church where Christ is preached and taught, and ask God to help you find new friends there who can encourage and help you. If you need further help, the pastor can point you to resources in your community.
It isn't necessarily easy to break habits like this - but with God's help, you can. He'll help you face your problems and deal with them, instead of trying to escape from them through drugs. Remember: "All things are possible with God" (Mark 10:27).