Is dianabol good or bad that i have bodybuilding? i have asked my trainer he said that is very bad !

Don't do it: It will help you build muscle. The results may be grotesque. Most women strongly prefer a natural bodybuilder. It's rough on your body overall, including vessels. Your testes will shrink spectacularly, and unless nobody sees them, everybody will know you're on steroids. It may affect your mind & make you do something you'll regret for the rest of your life. And who likes a cheater / lawbreaker?