Insure and Ensure are two words that I see mixed up with alarming frequency.
Insure means "to provide or obtain insurance for." Some correct usages of this word would be:
I can't drive my car until it's been insured.
You should insure your house against flood damage.
Ensure means "to make sure or certain." Examples:
If he comes with us, I cannot ensure his safety.
Don't leave until you've ensured that all the cages are locked.
Basically, insure should only be used if you're speaking of obtaining some literal form of insurance. If you could swap out the word with guarantee or make sure, than you should go with ensure.
Some dictionaries disagree with the definitions I've listed here. For instance, the Merriam-Webster dictionary states that insure can be used to mean the same thing as ensure.
The reason for this is that language is constantly changing. In this case, insure and ensure have been mixed up so much that many dictionaries are essentially throwing up their hands and saying, "Fine! If you're all going to misuse insure, we'll just change the definition to include how you're using it." It's the same thing that's happening with the word literally: so many people use it to mean figuratively but with hyperbolic emphasis that many dictionaries are accepting this as a new, legitimate use of the word.
But that doesn't mean that you can discount the definitions I've given you here. See, just because many people are using a word in a new way does not mean that they sound intelligent or professional when they do. Suppose you submitted to an editor who really dislikes the way that people are using insure to mean ensure. If that editor sees that you've used insure that way in your manuscript, it will stand out to them as an error, as a sign that you don't know what you're doing. Will they reject your manuscript over that? Probably not. But it could be the straw that breaks the camel's back. You're better off playing it safe and following the definitions I've given you here.