Fasting is doing without something for a specific amount of time to focus on a spiritual need in your life. Why are you fasting? That should be the first thing you decide. Do you have a difficult decision to make? Do you struggle with a specific sin in your life? Do you need some spiritual direction or encouragement? Whatever it may be, you first need to determine why you are doing it.
Once, you decide why you are fasting, then, you can decide what you are fasting. Fasting should never be seen as a diet plan or way to rid yourself of something that you want to stop. Your fast should be removing something in your life that is a hindrance from your devotion to God.
Much like "giving up something for Lent" you should look into sacrificing something significant that will cause you to depend more on God. Many people give up sodas, chocolate, sweets, television or even social media. What is it that the Holy Spirit is telling you distracts you from being in God's presence?
Finally, after you have decided why and what, you need to find someone to tell. It could be your good friend, d-group leader or youth staff. But, you need to find someone whom you trust to tell about your commitment. Ask this person to help keep you on track.
There are many things you can fast from, including food. Here is a link to the Daniel Fast which is a great one to try.
The Daniel Fast