Boiling water is one of the safest ways to purify it. Water should be boiled for one full minute at a rolling boil. If you can not boil your water, using bleach is another alternative for purification.
Normal household bleach is usually between 5-6% chlorine and can be used following the chart guidelines below. Do not use perfumed, dyed bleach that has any additives. Use plain bleach as follows for water purification.
Source: Washington Dept of Health
Use a clean container and mix thoroughly and allow to stand for at least 30 minutes before using (60 minutes if the water is cloudy or very cold).