Formal:
* Je crois: This is the most common and literal translation, meaning "I believe."
* Je suis convaincu(e): This translates to "I am convinced" and is used when you are very sure of your belief.
Informal:
* Je pense: This translates to "I think," and can be used interchangeably with "je crois" in informal settings.
* Je suis d'accord: This translates to "I agree," and is used when you share someone else's belief.
Other considerations:
* Je suis sûr(e) que...: This translates to "I am sure that..." and is used when you are very confident in your belief.
* J'ai la conviction que...: This translates to "I have the conviction that..." and emphasizes your strong conviction.
The best choice depends on the specific situation and the level of formality you desire.