据说 + (,) + [clause]
[subject] + 据说 + [verb phrase]
据 + [person] + 说 + ,+ [clause]
据说 reports hearsay: the speaker passes on a claim without vouching for it or naming the source. It usually opens the sentence, and the comma after it is optional.
It can also follow the subject and sit before the verb phrase.
To name the person, split the word: 据 + person + 说.
听说 says the speaker heard it personally, and 我 can go before it. 据说 is impersonal and never takes 我. 据说 suits legends, history and general rumour; 听说 suits news passed on by people you know.
Formal news avoids 据说 and names its source with 据…报道, the 据 point.