Telling your 9 month old No!!

I'm a first time mom and I know babies will get into things they shouldn't but my baby girl loves the fireplace doors and the entertainment center.....my question is any one have tips on what they are doing to teach their babies NO that's not allowed. I know they are still young and don't know the difference but I know I want to start now even if I have to do it 20 million times a day because I know at some point they will understand that it's a No No!!! Any tips???