unauthorized charge

I looked at my bank account this morning and discovered a charge of $74.90 which was not authorized by me.  I'm trying to figure out why I'm being charged and my bank says that it looks like it could possibly be coming out every month.  Now I have to "dispute" this charge, and in doing so, it will immediately cancel my current card, and issue a new card which puts me in a disadvantaged situation for the next week or so, until the new card arrives.  Not happy over this situation.