Correct me if I'm wrong but I think the answer is: Journalism is an effective tool, for it develops new essay topics.
Journal writing can be shortened to journalism. Also, the are needs to be an is, and since you said to put a comma after tool, I'm guessing "for" is a conjunction. Coordinating conjunctions are used in compound sentences, so I added a subject to the second independent clause and matched the tense to the first clause.
