Obviously the point of fasting is penitential.
You're arming yourself for spiritual warfare; strengthening your soul so it's not a quivering mass of jelly, but turning into something stronger, with God's grace.
Deny yourself: if you don't already eat meat, or only eat minimally as it is, or are exempt from the traditional laws of fasting, "fast and abstain" from something else. Coffee, milk, condiments on your food, butter, etc. Give up something you really enjoy doing--reading, for example. The harder the better.