"I believe the company is bad."
Translation:저는 회사가 나쁘다고 믿어요.
Your sentence has a similar meaning but translates to "The company seems bad". It's not as strong as saying "I believe".
The grammar works like any other quote, like 엄마가 춥다고 했어 - mom said it's cold. ~다고 하다 is quoting someone's statement (not commands or questions). You can use the same ~다고 ending for the verb and add 믿다 or 생각하다 for I believe/I think (x).