Why does my bread burn at the bottom before it is done?
It is highly likely that the temperature of your oven is too high.  While I would recommend simply buying an oven thermometer, which are not very expensive, if you are in a part of the world where that is not feasible, according to this post at Tip King:


  You can check your oven's temperature with this simple test. After
  preheating it, lay a sheet of plain white paper on the centre rack and
  leave it for five minutes. The paper's colour will tell you the oven's
  temperature:
  
  
  pale biscuit, 150&deg;C/300&deg;F or less
  light brown, 180&deg;- 200&deg;C/350&deg;-400&deg;F
  golden brown, 200&deg;- 230&deg;C/400&deg;-450&deg;F
  dark brown, 230&deg;- 260&deg;C/450&deg;-500&deg;F
  black, over 260&deg;C/500&deg;F
  


Hopefully, this method will let you estimate the temperature of your oven accurately enough to adjust it within the range that your bread is successful.

Note that I have not verified this method personally.

If this is not the cause of your problems, you will need to describe how you are baking your bread (the recipe, whether you are using a stone, a pan, how you are shaping the loaves, what their size is, and so forth) in much more detail to get a better answer.