Do I spell it offence or offense?

There are a variety of words that seem to be spelled really similarly--with maybe just one letter different, and it often causes confusion when you're trying to figure out which is the correct spelling.  When I was teaching English in Mexico, students often asked me if they were spelling words like "colour" incorrectly because they would spell it "color."  It was a great teachable moment to point out the difference between British and American spellings.  The good news is that "offense" and and "offence" have the same explanation.  Offense is the preferred American spelling, and offence seems to be preferred everywhere else.

However, you may be wondering if words like practise and practice or advise and advice are also like the offense/offence situation.  In this case, they are not.  Practise and advise are verbs, and practice and advice are nouns, so you'll have to make sure you use them appropriately if you're wondering which spelling to use.