When words like "I believe" and "I think" begin a sentence, they are functioning as the subject and verb in the sentence and take no punctuation.
...I believe he called in a little before 3:00.
...I think he was with us for ten years.
When they are in the middle of the sentence, they are parentheticals and need commas around them.
...He called in, I believe, a little before 3:00.
...He was with us, I think, for ten years.
Happy punctuating!
Margie