First of all, you need to prepare for the fast. Try to eat a healthy meal before you start your fast. Having nothing to eat and staying hungry for hours can affect your health in a big way so it is extremely important to have a decent snack.
Set the time
If you are keeping the fast which is set by your religion, there will certainly be a time limit. However, if you are keeping the fast for a specific act it is important that you set a time for it. Muslims fast from sunrise to sunset which is a good 12-15 hours. Your fast should also be for approximately the same time period otherwise it will be of no use.
It is extremely important that you practice your religion and spirituality during the fast. You must abstain from all bad activities and try to pray for as long as you can. Give yourself time to read the Holy Quran if you are a Muslim, Holy Bible if you are a Christian and Torah if you are a Jew.
End the fast by praying
The fast should not end abruptly and you should not just start eating. Have some patience and pray for whatever purpose you had kept the fast. Pray for yourself and your loved ones and ask for forgiveness from the Almighty. After you have done that, start to eat slowly.