If you're trying to ask someone's opinion about a film, then you can use either of the following sentences:
What did you think of the film? (I would also normally use the past tense.)
How was the film?
In the first case, I would use "what" in the same way that I would use "what" in the following sentence:
What was your opinion of the film?
I can imagine a situation in which you would start a sentence with "How did you think...", but it would be extremely uncommon:
* How did you think about the film? (Note the use of "about" instead of "of".)
I would interpret this question to mean: "What was the thought process you used in analyzing the film?"
@Matt points out that this sentence is unidiomatic, and I would agree. The only context in which I would expect to hear it would be a film studies class in which the professor was asking me to explain the thought process I used in analyzing a film. Even then, there are probably clearer ways to express this thought.