A difference in meaning exists between these two words:
INSURE has to do with money changing hands: You insure your house against fire, flood and earthquakes. You insure a package at the post office. You insure your car against damages.
ENSURE means to “make certain.” By setting your alarm, you ensure you will not be late for your early morning flight. By reading a recipe before starting to cook, you ensure that you have all the ingredients and equipment necessary. By drinking Ensure, you ensure you will get adequate vitamins and calories. (This is not a paid announcement and I’ve never drunk Ensure, but I’m guessing that’s its purpose.)