Can I skip "in" from "in which" at the beginning of the sentence?
No, not in this sentence. If you removed "in," the sentence would sound odd. It would be best if you had that preposition to connect the sentence. Here is the sentence without in: "Now ransomware, ah, you should know, is a type of cyber attack which the hacker actually locks up your data." However, you could use "where" instead of "in which," and the sentence stilly conveys the same meaning.