Fasting

I find it so hard to fast... Like I'm not even sure what I'm meant to do during the fast, what I'm meant to pray for, my mind drifts off elsewhere, and I don't know what I'm meant to do when I'm not praying like do I read the Bible? ....Do I drink juice or just water? How long do I do it for... Any tips? I tried it once and I prayed for an hour and got distracted :(