Hi Lilian! It's NOT a dumb question. It's a question that gets asked a lot!
There are so many certifications out there (over 300), it's easy to get confused.
They important thing is to find a reputable one; one that potential clients and gym owners who have done their research, will respect.
Read everything you can get your hands on as well. None of the courses can possible teach you everything!
The one John mentioned above, WITS (World Instructor Training Schools) is a good, up-and-coming cert! I happen to be an instructor for them, but I would not teach for them if I didnt believe they were a legit certifying body.
Feel free to PM me if you have any specific questions!
Wes